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PLATE I. 



ELECTEIC POWEE 
TEAI^SMISSION 



A PRACTICAL TREATISE 
FOR PRACTICAL MEN 



LOUIS BELL, Ph. D. 

MEMBER AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS 



FIFTH EDITION 
REVISED AND ENLARGED 



NEW YORK 

McGEAW PUBLISHING CO. 
1907 



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Copyright, 1906, by 

McGRAW PUBLISHING CO. 

New York 




PREFACE TO FIRST EDITION. 



) This volume is designed to set forth in the simplest possi- 

ble manner, the fundamental facts concerning present prac- 
tice in electrical power transmission. 

Busy men have little time to spend in discussing theories 
of which the practical results are known, or in following the 
derivation of formulae which no one disputes. The author 
has therefore endeavored, in introducing such theoretical 
considerations as are necessary, to explain them in the most 
direct way practicable ; using proximate methods of proof 
when precise and general ones would lead to mathematical 
complications without altering the conclusion for the purpose 
in hand, and stating only the results of investigation when 
the processes are undesirably complicated. 

In writing of a many-sided and rapidly changing art, it is 
impossible in a finite compass to cover all the phases of the 
subject or to prophesy the modifications that time will bring 
forth ; hence, the epoch of this work is the present, and the 
point of view chosen is that of the man, engineer or not, who 
desires to know what can be accomplished by electrical power 
transmission, and by what processes the work is planned and 
carried out. This treatment is not without value to the stu- 
dent who wishes to couple his investigations of electrical 
theory with its application in the hands of engineers, and puts 
the facts regarding a very great and important development 
of applied electricity in the possession of the general reader. 

Such apparatus as is described is intended to be typical of 
the methods used, rather than representative of any particu- 
lar scheme of manufacture or fashion in design. These last 
change almost from month to month, while the general con- 
ditions remain fairly stable, and the underlying principles 
are of permanent value. 

January, 1897. 



PREFACE TO FIFTH EDITION. 



The art of electrical power transmission has changed but 
little in the past two years, since the fourth edition of this work 
went to press. There have been very many plants installed, 
few of them at all sensational in magnitude, voltage, or distance 
of transmission. The great bulk of such work is now rather 
commonplace. The upper limit of voltage has already risen to 
nearly 70,000 volts, and the next few years will assuredly see a 
very material increase over this figure. A few new pieces of 
apparatus have been recently brought into use, which have been 
noted in their appropriate places. Perhaps the most considera- 
ble impending changes are those in the resources of electric 
lighting which affect only those transmission systems which do 
their own distribution. These changes, however, bid fair to be 
on a very large scale and of very striking character within the 
next few years. 

Juney 1907. 



CONTENTS. 



Chapter. Page. 

I. Elementary Principles 1 

II. General Conditions of Power Transmission ... 23 

III. Power Transmission by Continuous Currents ... 77 

IV. Some Properties of Alternating Circuits .... 125 

V. Power Transmission by Alternating Currents . . 158 

VI. Alternating Current Motors 217 

VII. Current Reorganizers 280 

VIII. Engines and Boilers 310 

IX, Water-Wheels 349 

X. Hydraulic Development 387 

XI. The Organization of a Power Station 418 

XII. Auxiliary and Switchboard Apparatus 455 

XIII. The Line 474 

XIV. Line Construction 536 

XV. Methods of Distribution 581 

XVI. The Commercial Problem 639 

XVII. The Measurement of Electrical Energy 660 

XVIII. High Voltage Transmission 687 



LIST OF TABLES. 



Page. 

Efficiency of Wire Rope Drives 39 

Wire Ropes and Pulleys Therefor 42 

Loss of Head in Hydraulic Pipes 47 

Loss of Air Pressure in Pipes 52 

Efficiencies of Electric Motors 62 

Efficiencies of Electric and Other Transmissions .... 74 

Performance of Small Polyphase Motors 270 

Steam Consumption of Engines 319 

Evaporative Power of Fuels 329 

Evaporative Tests of Boilers 330 

Coal Consumption of Engines 332 

Table for Weirs 391 

Properties of Steel Hydraulic Pipe 408 

Properties of Copper and Other Wires 486 

Size, Resistance, and Weights of Copper Wires 509 

Natural Tangents, Sines, and Cosines 522 

Size, Weight, and Tensile Strength of Line Wires .... 539 

Sizes and Weights of Wooden Poles 551 

Tensile Strength of Woods 554 

Properties of Direct and Alternating Current Arcs . . . 592 

Cost of Power with Various Engines 640 

Cost of Intermittent Power with Various Engines .... 643 

Cost of Electric Motors 644 

Typical List of Discounts 657 

List of American Transmission at or above 20,000 Volts . 704 



ELECTRIC TRANSMISSION OF POWER. 



CHAPTER I. 

ELEMENTARY PRINCIPLES. 

It has long been the fashion to speak of what we are 
pleased to call electricity as a mysterious ''force/*' and to 
attribute to everything connected with it occult characteristics 
better suited to mediaeval wizardry than to modern science. 
This unhappy condition of affairs has, in the main, come about 
through the indistinctness of some of our fundamental ideas 
and inexactitude in expressing them. 

To speak specifically, there has been, even in the minds 
and writings of some who ought to know better, a tendency 
toward confusing the extremely hazy individuality of ''elec- 
tricity" with the sharply defined properties of electrical 
energy. We have been so overrun by theories of electricity, 
two-fluid, one-fluid, and non-fluid — by electrically "charged" 
atoms and duplex ethers, that we have well-nigh forgotten the 
very great uncertainty as to the concrete existence of elec- 
tricity itself. Even admitting it to be an entity, it most 
assuredly is not a force, mysterious or otherwise. Electrical 
force there is, and electrical energy there is, and with them 
we can freely experiment, but for most practical purposes 
"electricity" is merely the numerical factor connecting the 
two. It is related to electrical energy much as that other 
hypothetical fluid "caloric" was supposed to be related to 
heat energy. The analogy is not absolutely exact, but it 
nevertheless summarizes the real facts in the case. 

The day has passed wherein we were at liberty to think of 
"electricity" as flowing through a material tube or as plas- 



2 ELECTRIC TRANSMISSION OF POWER. 

tered upon bodies like a coat of paint. The things with which 
we have now to deal are the various factors of electrical 
energy. 

It is the purpose of this chapter to treat of that form of 
energy which we denominate electrical, to discuss its relation 
to other forms of energy and some of the transformations 
which they may reciprocally undergo. 

Speaking broadly, energy is power of doing work. The 
energy of a body at any moment represents its inherent capacity 
for doing work of some sort on other bodies. This, however, 
must not be understood as implying that the aforesaid energy 
is limited by our power of utilizing it. We may or may not be 
able to employ it to advantage or under possible conditions. 
As an example, take the massive weight of a pile driver. 
Raised to its full height it possesses a certain amount of gravi- 
tational energy — a possibility of doing useful work. This 
energy is temporarily unemployed and appears only as a stress 
on the supporting rope and frame- work. Under these cir- 
cumstances, wherein the energy exists in static form, it is 
generally known as potential energy. 

Now let the weight fall and with swiftly gathering velocity 
it strikes the pile and does work upon it, settling it deep into 
the mud. The energy due to the blow of the moving weight, 
energy of motion in other words, is called kinetic. But at the 
bottom of its fall the weight still has potential energy with 
reference to points below it, and we realize this as the pile 
settles lower and each successive blow becomes more forceful. 
At some point we are unable further to utilize the fall, and 
have then reached the limit of the available energy in this par- 
ticular case. 

We must not forget, however, that each time the weight 
was lifted, work had to be done against gravitation to give the 
weight its point of vantage with respect to available energy. 
This work was probably done by utilizing the energy of 
expanding steam — in other words, the energy of the steam 
was transformed through doing work on the piston into kinetic 
energy of the latter, which, through doing work against gravi- 
tation, has been enabled again to reappear as the energy of 
a falling body, and to do work on the driven pile. And back 



ELEMENTARY PRINCIPLES. 3 

of the steam energy is the heat energy, by which work is 
done on the water in the boiler, and yet back of this the chemi- 
cal energy of the coal, transformed into heat energy and doing 
work on the minute particles of iron in the boiler, for we know 
that heat is a species of kinetic energy. 

Even the work done on our pile is not permitted to go un- 
transformed into energy. Part is transformed into heat energy 
through friction and compression of the pile, part through fric- 
tion of the water, and part raises ripples that may lift against 
gravity chips and pebbles on a neighboring shore. Other frac- 
tions go into the vibrational energy of sound; into heating the 
weight so that it gives out warmth — radiant energy — to the 
hand when held near it and to the surrounding air; and into 
electrical work done on the weight and neighboring objects, for 
the weight unquestionably receives a minute amount of elec- 
trical energy at each blow. Thus, a comparatively simple 
mechanical process involves a long series of transformations 
of energy. 

No energy is ever created or destroyed, it merely is changed 
in form to reappear elsewhere, and work done is the link 
between one form of energy and another. And we may lay 
down another law of almost as serious import: No form of 
energy is ever transformed completely into any other. 

On the contrary, the general rule is that with each transfor- 
mation several kinds of energy appear in varying amounts, and 
among them we may always reckon heat. The object of any 
transformation is usually a single form of energy, hence practi- 
cally no such thing as perfectly efficient transformation can be 
obtained. The energy by-products for the most part cannot 
be utilized and are frittered away in useless work or in storing 
up kinds of potential energy that cannot be employed. 

The greatest loss is in heat, which is dissipated in various 
ways and cannot be recovered. The presence of unutilized 
heat always denotes waste of energy. 

From what has gone before, we can readily appreciate that 
when we do work with the object of rendering available a 
particular kind of energy, the method must be intelligently 
selected, else there will result useless by-products of energy 
which will seriously lower the efficiency of the operation. 



4 ELECTRIC TRANSMISSION OF POWER. 

Whenever possible we utilize potential energy already exist- 
ing in securing a transformation. Thus if heat is wanted, the 
easiest way of getting it is to burn coal, and to allow its energy 
to become kinetic as heat. If we want mechanical work done, 
we set heat energy to work in the most efficient way practi- 
cable. If electrical energy is desired, we set the energy of 
steam to revolving the armature of a dynamo. If the right 
method of transformation is not chosen, much of the energy 
will turn up in forms that we do not want or cannot utilize. 
Burning coal is a very bad way of getting sound, just as play- 
ing a cornet is but a poor means of getting heat, although a 
fire does produce a trifling amount of sound, and a cornet by 
continual vibration must be warmed to a minute degree. 

These seem, and perhaps are, extreme instances, but when 
we realize that, somewhat to the discredit of human ingenuity, 
less than one-twentieth of the electrical energy supplied to an 
incandescent lamp appears in the form of light, the comparison 
becomes grimly suggestive. 

Understanding now that in order to obtain energy in any 
given form (such as electrical), particular methods of transfor- 
mation must be used in order to secure anything like efficiency, 
we may look a little more closely at various types of energy to 
discover the characteristics that may indicate efficient methods 
of transformation, particularly as regards electrical energy. 

Speaking broadly, one may divide energy into three classes: 

1st. Those forms of energy which have to do with move- 
ments of, or strains in, masses of matter. In this class may 
be included the ordinary forms of kinetic energy of moving 
bodies and the like. 

2d. Those which are concerned with movements of, or 
strains in, the molecules and atoms of which material bodies 
are composed. In this class we may reckon heat, latent and 
specific heats, energy of gases, and perhaps chemical energy. 

3d. All forms of energy which have to do with strains which 
can exist outside of ordinary matter, i.e., every kind of radiant 
energy and presumably electrical energy. 

These classes are not absolutely distinct; for example, we 
do not know the relation of chemical energy to the third class, 
nor of gravitational energy to any class, but such a division 



ELEMENTARY PRINCIPLES. 6 

serves to keep clearly in our minds the kind of actions to 
which our attention is to be directed. 

It is only within the past few years that we have been able 
with any certainty to classify electrical energy, and even now 
much remains to be learned. For a very long while it has 
been known that light, i.e., luminous energy, must be propa- 
gated through a medium quite distinct from ordinary matter 
and possessing certain remarkable properties. It was well 
known that luminous energy is transferred through this 
medium by vibratory or wave motion. Even the period of 
the vibrations and the lengths of the waves were accurately 
measured, and from these and similar measurements it has 
been possible to classify the mechanical properties of this 
medium, universally called ''the ether," until we really know 
more about them than about the properties of many kinds of 
ordinary matter — a number of the rare metals, for example. 

The next important step was the discovery, verified in the 
most thorough manner, that what had been known as radiant 
heat, such as we get from the sun or any very hot body, is 
really energy of the same kind as light. That is, it was found 
to be energy of wave motion of precisely the same character 
and in the same medium, differing only in frequency and wave 
length. It also has turned out in similar fashion that what 
had been called ''actinic" rays, that are active in attacking a 
photographic plate and producing some other kinds of chemi- 
cal action, are only light rays of shorter wave length than 
usual, and so ordinarily invisible to the eye. 

So much having been ascertained, it became clear that 
instead of three kinds of energy — "heat, light, and actinism," 
we were really dealing with only one — radiant energy, vibrat- 
ing energy in the ether, varying in effect as it varies in fre- 
quency. Speaking in an approximate way, such wave energy 
has a frequency of six hundred thousand hillion vibrations per 
second and a velocity of propagation of about a hundred and 
eighty-five thousand miles per second, so that each wave is 
not far from one fifty-thousandth of an inch long. These 
dimensions are true of light waves; chemical action can be 
produced by waves of half the length, while so-called heat 
rays may be composed of waves two or three times as long as 



6 ELECTRIC TRANSMISSION OF POWER. 

those of light. Such figures are starthng, but they can be 
verified with an accuracy greater than that of ordinary 
mechanical measurements. 

We see that this radiant energy is capable of producing 
various disturbances perceptible to our senses, such as chemi- 
cal action^ light, and heat, and that these different effects 
simply correspond to waves of energy having different fre- 
quencies and wave lengths. This being so, it is not imnatural 
to suppose that at still different frequencies other effects 
might be noted. This idea gains further probability from the 
experimental fact that waves of very different frequency 
traverse the ether with precisely the same velocity, showing no 
signs of slowing down or dying out, so that there seems to be 
no natural limit to their length. 

During the past half dozen years it has been clearly shown 
that ''radiant energy" is capable of producing profound 
electrical disturbances, such as violent oscillations of electrical 
energy in conducting bodies, and that these effects exist what- 
ever the frequency of the ether waves concerned. This verj' 
important fact was clearly foreseen by Maxwell more than 
twenty years ago, regarding light, and his prediction has been 
thoroughly verified through the persistent researches of the 
late Professor Hertz and others. 

This discovery is often expressed by saying that radiant 
energy is an electro-magnetic disturbance, or that light is one 
kind of electrical action. It is more strictly accurate to say 
that radiant energy, just as it produces chemical disturbances 
on the photographic plate, affects the eye as light, and material 
bodies as heat, is also capable of producing electrical effects 
when transferred to the proper media. Most of our experi- 
ments on its electrical effects have been performed with waves 
many thousand times longer than those of light, but their gen- 
eral character has proved to be exactly the same. 

A given substance may be differently related to waves of 
radiant energy of different lengths, but the phenomena are 
still essentially the same. For instance, a plate of hard 
rubber is thoroughly opaque to waves of a length correspond- 
ing to light, but is quite transparent to those of considerably 
greater length, such as can produce thermal or electrical 



ELEMENTARY PRINCIPLES. 7 

effects. A plate of alum will let through light waves and very 
long waves, but will stop most of those which are efficient in 
producing heat. A thick sheet of metal is quite opaque to all 
known waves of radiant energy. Hence the fact noted long 
ago by Maxwell, that all good conductors are opaque to light, 
although the converse is not true. 

The substance of all this is, that the same sort of disturb- 
ance in the ether which produces light is also competent to set 
up electrical actions in material bodies, and conversely, such 
actions may and do produce corresponding disturbances in the 
ether, which are thus transferred to other bodies. Such a 
transference corresponds to all that we know concerning the 
velocity with which electrical and electro-magnetic disturb- 
ances pass from body to body. It is equally certain that this 
velocity totally transcends anything we could hope to obtain 
from bodies having the dynamical properties of ordinary 
matter, while it does fit exactly the dynamical properties of 
the ether. 

We are thus forced to the conclusion that when an electrical 
current, as we say, '^ passes along" a wire, whatever a ''cur- 
rent" may be, it is not siniply transferred from molecule to 
molecule in the wire as sound or heat would be, but that there 
is an immensely rapid transfer of energy in the neighboring 
ether that reaches all points of the wire almost simultaneously. 
It takes a measurable time for the electrical energy to reach 
and utilize the centre of the wire, although its progress along 
the surface, thanks to the free ether outside, is enormously 
rapid. 

Thus takes place what is generally called a ''flow of elec- 
tricity" along the wire. Looking at the process more closely, 
the nearest approach to flow is the transfer of energy along 
the wire by means of stresses in the ether which in turn set up 
strains in the matter along their course. 

Whenever we cause in matter the particular stress which we 
call electromotive force for lack of a more exact name, the 
resulting strain is electrification, and if the stress be applied 
at one point of a conducting body, the strain is immediately 
transferred to other points by the stresses and strains in the 
surrounding ether. Wherever this transference of strain exists 



8 ELECTRIC TRANSMISSION OF POWER. 

we have an electrical current, although this name is generally 
reserved for those cases in which there exists a perceptible 
transference of energy by the means aforesaid. If the condi- 
tions are such that energy must be steadily supplied to keep 
up the electromotive stress, we have such a state of things as 
we find in a closed circuit containing a battery. 

To cause such a flow of energy we must first find means of 
setting up electromotive stress capable of being propagated 
through the ether. Now atoms and molecules are the only 
handles by which we can get hold of the ether. Only in so 
far as we can work through them can we do work on the 
ether. 

As a matter of fact, we cannot do work of any kind on the 
molecules of a body without setting up electrical stresses of 
some sort. In most cases of mechanical work, which in the 
main produces stress on the molecules only by strains in 
the mass, the energy appears mainly as heat, and is only inci- 
dentally electrical, as for instance the energy wasted in a 
heated journal. 

When, however, by any device we do work more directly on 
the molecules of a body, or on the atoms which compose the 
molecules, we are more than likely to transform much of this 
work into electrical energy. As a rough example of the two 
kinds of action just mentioned, pounding a body heats it with- 
out causing any considerable electrification, while on the other 
hand rubbing it rather gently, sets up a considerable electrifica- 
tion without heating it noticeably. 

In fact, for many centuries, friction was the only known 
m.ethod of causing electrification. Later, as is well known, it 
was discovered that certain sorts of chemical action, which has 
to do directly with interchanges of energy between molecules, 
were very potent in electrical effects. With this discovery 
came the abihty to deal with steady transfers of electrical 
energy in considerable amount (electric currents), instead of 
the relatively slight and transitory effects previously known 
(electrification, ''frictional'' electricity). 

To clear up the real nature of this difference it is well to 
consider what we mean by saying that a body is electrified, or 
has an electrical charge. In other words, what is electrifica- 



ELEMENTARY PRINCIPLES. 9 

tion? Not very many years ago this question would have 
been answered by saying that a quantity of a substance, posi- 
tive electricity (or negative as the case might be), had been 
communicated to the body in question; that this remarkable 
substance could reside only at the surface of the body and 
was able to produce in surrounding bodies exactly an equal 
quantity of negative electricity; that this ''charge" of elec- 
tricity would repel another ''charge" of the same substance 
placed near it, or attract a charge of its opposite, the other 
substance called negative electricity; and much more to the 
same effect. All this was a very convenient hypothesis — it 
explained, after a fashion, the common facts and enabled 
investigators to discover many important electrical relations 
and laws. But it expressed much more than there was any 
reason to know. From the standpoint of our modern doc- 
trines of energy, electrification is a very different thing. 

Let an electromotive stress (from whatever source) be 
applied to a body, a metallic sphere for example, long enough 
to transfer to it a finite amount of energy. This energy 
appears as stresses and strains in the ether everywhere about 
the body under consideration and thence extends to the mole- 
cules and atoms of neighboring bodies, causing "induced 
charges." It is as if one were to fill a box with jelly, and then 
pull or push or twist a rod embedded in its centre. The 
result would be strains in the rod, the jelly, and the box, and in 
a general way the total stress on the box would equal that on 
the rod. By proper means we could detect the strain all 
through the substance of the jelly, but most easily by its varia- 
tions from place to place. 

We do not know exactly what sort of a strain in our ether 
jelly is produced by electromotive stress, but we do know that 
it possesses the quality of endedness, so that the strains in 
the matter concerned, i.e., in the ball and surrounding bodies, 
are equal and opposite. 

In fact, the two "charges" are in effect the two ends of the 
same strain in the ether. They appear to us to be real attri- 
butes of the two opposed surfaces, because at these surfaces 
the dynamical constants, such as density, elasticity, etc., of 
the medium through which the strain is propagated, change 



10 ELECTRIC TRANSMISSION OF POWER. 

in value, and differences in state of strain there become physi- 
cally manifest. 

In electric currents we have a very different state of things. 
The energy supplied by the electromotive stress, instead of 
becoming potential as electrostatic strain, and producing 
''charge," does work and is transformed into other kinds of 
energ}^, thermal or chemical, mechanical or luminous. 

When a stress of whatever kind is applied to a body, only a 
limited amount of energy can be transferred by it so long as 
the energy remains potential. Thus, in our box of jelly before 
referred to, a twist of given intensity applied to the stick, as 
for instance by a string wound around it and pulled by a given 
weight, can only transfer energy until the stresses produced in 
the jelly come to an equilibrium with it. On the other hand, if 
the box were filled with water and the stick were the axle of a 
sort of paddle wheel, the very same intensity of twist could go 
on communicating energy to the water as long as one chose 
to apply the necessary work. 

This roughly expresses the difference between electric 
charge and electric current, viewed from the standpoint of 
energy. An electromotive stress applied to a wire charges it 
and then the transfer of energy ceases. If the same stress be 
applied under conditions that allow work to be done by it, 
energy will be transferred so long as the stress is kept up. In 
an open electric circuit we have a charge as the result of elec- 
tromotive stress. When the circuit is closed, i.e., when a 
continuous medium is furnished on which work can be done, 
we have an electric current. The amount of this work and 
the flow of electrical energy that produces it depend on the 
nature of the circuit. Certain substances, especially the 
metals, and of metals notably copper and silver, permit a 
ready continuous transfer of energy in and about them. Such 
substances are called good conductors. The real transfer 
of energy takes place ultimately via the ether, but its amount 
is limited by the amount and character of the matter through 
which work can be done. 

Whenever the strains in the ether, such as we recognize in 
connection with electrical charge, shift through space as when 
a current is flowing, other strains bearing a certain relation 



ELEMENTARY PRINCIPLES, 11 

to the direction of flow are made manifest. Where there 
is a rapid and intense flow of energy, these strains are very 
great and important compared with any electrostatic strains 
that exist outside the conducting circuit. In other cases they 
may be quite insignificant. These strains are electro-magnetic, 
and with them we have to do almost exclusively in practical 
electrical engineering. They appear wherever there is a moving 
electrical strain, whether produced by moving a charged body 
or causing the charge upon a body to move. 

Both kinds of strains exist in radiant energy, as in other 
cases of flowing energy. The stresses in electro-magnetic 
energy are at right angles both to the electrostatic stresses 
and to the direction of their motion or flow. If, for example, 





Fig. 2. 



we have a flow of electrical energy in a straight wire (Fig. 1), 
the electro-magnetic stresses are in circles about it. 

If A be a wire in which the flow of energy is straight down 
into the paper, the electro-magnetic stresses are in circles in 
the direction shown by the arrow heads. If the wire be bent 
into a ring (Fig. 2), with the current flowing in the direction of 
the arrows, then the electro-magnetic stresses will be (follow- 
ing Fig. 1) in such direction as to pass downward through the 
paper inside the ring. 

These electro-magnetic stresses constitute what we call a 
magnetic field outside the wire. The intensity of this field can 
be increased by increasing the flow of energy in the desired 
region in the systematic way suggested by Fig. 2. If, for 
example, we join a number of rings like Fig. 2 into a spiral 
coil shown in section in Fig. 3, in which the current flows 



12 



ELECTRIC TRANSMISSION OF POWER. 



downward into the paper in the lower edge of the spiral, 
there will be produced a magnetic field in which the stresses 
have the direction shown by the arrows. Such a spiral consti- 
tutes a genuine magnet, and if suspended so as to be free 
to move would take up a north and south position with its 
right-hand end toward the north. In and about the spiral 
there exists a magnetic ''field of force," which is merely another 
way of saying that the ether there is under electro-magnetic 
stress. Its condition of strain is closely analogous to that 
about an electrified body, and, as in that case, there is no 






I fbu 



^ / 



b 



)) 



Fig. 3. 

work done on the ether after the strains are once established, 
since the energy then becomes potential. While this is being 
accomplished, work is done just as when a body is charged. 

If, now, setting up such an electro-magnetic field requires 
energy to be spent by causing a current to flow in the spiral, 
we should naturally expect that if the same field could be set 
up by extraneous means, energy would momentarily be spent 
on the spiral in producing stresses and strains similar to those 
that set up the original field. This is found to be so, the 
process working backward as well as forward. 

If, for example, we have two rings (Fig. 4), and by sending 
a current around one, transfer energy to the medium outside 
it, this energy will set up an electromotive stress in the other 
ring. The direction of this stress is not at once obvious, but 
we can get a very clear idea of it by considering the work 
done. If current is started in A (Fig. 4), in the direction 
shown, electro-magnetic stresses are produced in the direction 



ELEMENTARY PRINCIPLES. 13 

of the arrow C. If these are to do work on B, the electro- 
motive stress in the latter cannot have such a direction as to 
set up on its own account a magnetic field that would assist 
that of A, otherwise we could increase the field indefinitely 
without added expenditure of energy. Therefore, the electro- 
motive stress in B, and hence the current, must be in a direc- 
tion opposing the original current in ^, as shown in the figure. 
In like manner if the current in A be stopped and the field 
due to it therefore changes, there are changes in the electro- 
magnetic stresses about B, that again set up an electromotive 
stress in it. If, however, this change of stress is to do work, 
the electromotive stress in B must be of such direction as to 




Fig. 4. 

oppose by its field the change in the field of A — i.e., it must 
change its direction and will now give us a current in the 
same direction as the original one in A. All this follows the 
general law, that if work is to be done by any stress it must 
be against some other stress. There can be no work without 
resistance. 

In Fig. 4 we have the fundamental facts of current induction 
on which depend most of our modern methods of generating 
and working with electrical energy. Summed up they amount 
to saying that whenever there is a change in the electro-mag- 
netic stresses about a conductor, work is done upon it, depend- 
ing in direction and magnitude on the direction and magnitude 
of the change in the stresses. 

This is equally true whether the stresses change in absolute 
value or whether the conductor changes its relation to them. 
Thus, in Fig. 4, if A carries an electrical current the result on 
B is the same whether the field of A changes through cessation 
of the current, or whether the same change in the stresses 



14 ELECTRIC TRANSMISSION OF POWER. 

about B is produced by suddenly pulling B away from A. The 
rate at which work is done depends on the rate at which the 
stresses are caused to change, as might be expected. So long 
as the stresses are constant with reference to the conductor in 
which current is to be induced, no work can be done upon it. 
These principles form the foundation of the dynamo, motor, 
alternating current transformer, and many other sorts of elec- 
trical apparatus. Their details may differ very widely, but we 
can get all the fundamental ideas from a consideration of Figs. 
3 and 4. To define somewhat the specific idea of the dynamo, 
consider what happens when a conducting wire is thrust into 
a magnetic field such as is produced by a coil, as in Fig. 5. 
As in Fig. 3, let the current in the coil be flowing down- 
ward into the paper in the lower half of the figure. ^ is a 
wire perpendicular to the plane of the paper in front of the 
coil, its ends being united at any distant point that is con- 



v.ilj 11 1; II i! E>' 



■ " t:^ 






Fig. 5. 

venient. Knowing that moving the wire into the field will set 
up electromotive stresses in it, we can as before determine 
their direction by remembering that work must be done. 
That is (see Fig. 1), the induced current will flow through A 
downward into the paper. In passing out of the field, the cur- 
rent would be upward. 

We have so far neglected the rest of the circuit. To be 
exact, we should consider it as in Fig. 6. Following the same 
line of reasoning as in Fig. 5, we see that while the ring A is 
entering the magnetic field the current induced in it must be 
opposite to that in the inducing coil (see Fig. 4). When the 
coil is leaving the field, however, this direction will be reversed. 
Considering the coil A as a whole, we see that so long as the 
total field tending to set up stresses in it is increasing, a cur- 
rent will be induced opposed to that in the inducing coil. 



ELEMENTARY PRINCIPLES. 



15 



While the total field is diminishing, the induced current will 
be in the other direction. The work that is spent in moving 
the coil A will for the most part reappear as electrical energy 
in that coil. Arrange the parts of Fig. 6 so that the motion 
of A can be accomplished uniformly and continuously, and 



R P, P P P P P Ir. "^^ 

|-tTt"'"^"f"*"++"M- + t 

f-+n-i-fi-t^rf^-f+-H 

|-f-t-f-t-r4-f4-^4-f4t 

j-1^-t-l-^t+-^++-f-+-^- — 

b b d d D b N-^^^tC 



Fig. 6. 

we should have a true, though rudimentary, dynamo. Such 
a structure could be made by fixing A to the end of an arm 
pivoted at the other end and then revolving the arm so that 
at each revolution the coil A would sweep through the field 
of the magnetizing coil (see Fig. 7). The result of this, as 
we have seen, would be on entering the field, a current in 
one direction, and on leaving, a current in the other. There 
would thus be an alternating current developed in the ring A, 




Fig. 7. 

If it were cut at some point, and wires led down the arm and 
to two metal rings on the axis 5, we could obtain, by pressing 
brushes on these rings, an alternating current in any outside 
circuit. To make more of the revolution of the arm useful, 
we could arrange inducing coils in a circle about B. There 
would then be an alternation as A passed each coil. 

All these devices, however, would produce comparatively 
weak effects, because it is difficult to produce powerful mag- 



16 ELECTRIC TRANSMISSION OF POWER. 

netic stresses in so simple a way. There are very few materials 
in which magnetic stresses are easily set up or propagated. 
Chief among these is iron, which bears somewhat the same rela- 
tion to magnetic actions that copper does to electrical ones. 
By giving to the coil in Fig. 7 a core of soft iron, the electro- 
magnetic effects obtained from it would be greatly enhanced. 
They are comparatively feeble in air, and the more iron we put 
in their path the better. Developing this idea, we have in 
Fig. 8 a much better device for setting up electric currents. 
Here the coil of Fig. 7 is wound around an iron core, the ends 
of which are brought near together. The arm of Fig. 7 is 




also of iron with enlarged ends, and the ring A is replaced by 
a coil of several turns. 

The magnetic stresses brought to bear on the coil A are thus 
made comparatively powerful. Following out on Fig. 8 the 
reasoning applied to Fig. 7, we see that considerable electro- 
motive stresses would be set up by the revolution of A, alter- 
nating in direction at each half revolution. In fact, A is the 
armature of a simple alternating dynamo, having two poles 
N and S, so called from their magnetic relations (see Fig. 3). 

We have not thus far considered the source of the electro- 
magnetic field involved. It may be obtained as shown by 
utilizing the electro-magnetic stresses set up by a wire convey- 
ing electrical energy, or on a small scale from permanent mag- 
nets. The essential fact, however, is that by forcing a wire 
through a region of electro-magnetic stress, electromotive 
stresses are set up in that wire, the action in every case being 
in such direction as to compel us to do work on the wire. 



ELEMENTARY PRINCIPLES. 17 

This work appears as electrical energy in the circuit including 
the moving wire. 

Now return to Fig. 5 and consider the effect if the wire A is 
carrying a steady flow of electrical energy. It will set up 
electro-magnetic stresses about it as already described. If 
the current be downward into the paper in A, these stresses 
will be opposed to the stresses in the field. Inasmuch as we 
have seen that in setting up such a current, work had to be 
done in forcing the wire into the field, it follows that given 
such a current, there must be between its field and that of the 
coil a repulsive force which had to be overcome by doing the 
work aforesaid. In other words, there must have been a 
tendency to throw A out of the field of the coil. Just as 
work had to be spent to produce electrical energy in A, so 
electrical energy will be spent in keeping up the stresses 
around A that tend to drive it out of the magnetic field. If 
the current in A were in the other direction, the stresses in its 
field and that of the coil would be concurrent instead of 
opposed, and their resultant would tend to draw wire and coil 
together, i.e., work would have to be spent to keep them 
apart. This is the broad principle of the electric motor. It 
is sometimes referred to as simply a reversal of the dynamo, 
but it really makes no difference whether the structure in 
which the action just described takes place is well fitted to 
generate current or not. Given a magnetic field and a wire 
carrying electrical energy, and there will be a force between 
them depending in direction on the directions of the electro- 
magnetic stresses belonging to the two. If either element is 
arranged so as to move and still keep up a similar relation 
of these stresses we have an electric motor. Whether so 
arranged as to fulfil this condition with alternating currents, 
or in such manner as to require currents in one direction only, 
the principle is the same. 

So far as unidirectional or '' continuous" currents are con- 
cerned they are usually obtained from dynamo electric machines 
similar in principle to Fig. 8. This machine, if the ends of 
the winding on the armature be connected to two metal rings 
insulated from each other, serves as a source of alternating 
currents which can be taken off the two rings by brushes 



18 ELECTRIC TRANSMISSION OF POWER. 

pressed against them. If it is necessary to obtain currents in 
one direction only, this can be readily done by reversing the 
connection of the outside circuit to the windings at the same 
moment that the current reverses in them. The simplest way 
of doing this is by a ''two part commutator/' such as is 
shown in diagram in Fig. 9. Here A is the shaft surrounded 
by an insulating bushing. On this are fitted two half rings, C 
and C, of metal (the commutator segments). On these bear 
brushes B and B\ If the ends of the winding are connected 
to C and C, and the brushes are so placed that they pass from 
one segment to the other at the moment when the current in 
the winding changes its direction, the direction of the current 




with respect to the brushes and the outside circuit with which 
they are connected obviously remains constant. 

In the actual practice of dynamo building very many refine- 
ments have to be introduced to serve various purposes, but the 
underlying principle remains the same, i.e., to set up in a con- 
ductor electromotive stresses by dragging it into and out of 
the strained region of ether under an electro-magnetic stress. 

According as the dynamo is intended for producing con- 
tinuous or alternating currents, its structure is somewhat 
modified with its particular use in view. These modifications 
extend not only to the general arrangement but to the details 
of the winding. Alternating dynamos usually have a more com- 
plicated magnetic structure than continuous current machines, 
and are almost invariably separately excited, i.e., have their 
magnetizing current supplied from a generator specialized for 
producing continuous current. The magnetic complication is 
really only apparent, as it consists merely of an increased num.- 



ELEMENTARY PRINCIPLES. 19 

ber of magnet poles, due to the desirability of obtaining toler- 
ably rapid alternations of current. 

Dynamos designed for producing continuous current are 
modified with the armature as a starting point. The winding 
is very generally much more complicated than that of an alter- 
nator, and the commutator that serves to reverse the rela- 
tion of the windings to the brushes at the proper moment is 
correspondingly elaborate. The magnetic structure is usually 
comparatively simple. The whole design is necessarily sub- 
ordinated to securing proper commutation. Continuous cur- 
rent dynamos are almost universally self-excited, that is, the 
current which magnetizes the field is derived from the brushes 
of the machine itself. Whatever the character of the machine 
the electromotive force generated in it increases with the inten- 
sity of the magnetic field (that is, with the magnitude of the 
electro-magnetic strains which affect the armature conductors), 
with the speed (that is, with the rate of change of electro- 
magnetic stress about these moving conductors), and with 
the number of turns of wire of which the electromotive forces 
are added. The capacity of the machine for furnishing elec- 
trical energy varies directly with the electromotive force and 
with the capacity of the armature conductors for transmitting 
the energy without becoming overheated. Practically all the 
energy lost in a dynamo appears in the form of heat, which 
must be limited to an amount which will not cause an undue 
rise of temperature. 

It is not the purpose of this chapter to deal with the prac- 
tical details of dynamo design and construction. For these, 
the reader should consult special treatises on the subject, 
which consider it with a fulness which would here be quite 
out of place. Special machines, however, will be briefly dis- 
cussed in their proper places and in relation to the work they 
have to do. 

Having now considered the principles which underlie the 
transformation of mechanical into electrical energy, we may 
profitably take up the fundamental facts in regard to the 
measurement of that form of energy and the units in which it 
and its most important factors are reckoned. 

All electrical quantities are measured directly or indirectly 



20 ELECTRIC TRANSMISSION OF POWER. 

in terms of the dynamical units founded upon the units of 
length, mass, and time. These derived dynamical units can 
serve alike for the measurement of all forms of energy, so 
that all have a common ground on which to stand. As the 
electrical units are derived directly from the same units that 
serve to measure ordinary mechanical effects, electrical and me- 
chanical energies are mutually related in a perfectly definite way. 

A natural starting point in the derivation of a working 
system of electrical units may be found in electro-magnetic 
stress, such as is developed about an electrical circuit or a. 
permanent magnet. To begin with, the mechanical units that 
may serve to measure any form of energy are derived from 
those of length, mass, and time. These latter are almost uni- 
versally taken as the centimetre, gramme, and second, the 
"C. G. S. " system. Starting from these the unit of force is that 
which acting for one second on a mass of one gramme can 
change its velocity by one centimetre per second. This unit is 
called the dyne, and as a magnetic stress it is equivalent to a 
push of about :i^-so-Qo of a pound's weight on a similar ''unit 
pole'' one centimetre distant. This unit is inconveniently small 
for practical use, and before long some multiple of it is likely 
to be given a special name and used for practical reference. In 
fact, one megadyne {i.e., 1,000,000 dynes) is very nearly equiva- 
lent to the weight of a kilogramme. Magnetic measure- 
ments may thus be made by direct reference to the dyne and 
centimetre, since the unit pole is that which repels a similar 
pole 1 centimetre distant, with a force of 1 dyne. 

Referring now to what has been said about the causes which 
vary the electromotive force produced in a dynamo, we fall at 
once into the definition of the unit electromotive force, which 
is that produced when field, velocity, and length of wire under 
induction are all of unit value. The unit electromotive force 
is, then, that which is generated in one centimetre of wire 
moving one centimetre per second, perpendicular to its own 
length, straight across unit field, which is that existing one 
centimetre from unit pole as indicated above. This unit, too, 
is inconveniently small, so that one hundred million times this 
quantity is taken for the practical unit of electromotive force 
and called the volt. 



ELEMENTARY PRINCIPLES. ^1 

The unit electrical current is that which flowing through one 
centimetre length of wire will create unit field at any point 
equidistant from all parts of the wire (as when the wire is 
bent to a curve of 1 centimetre radius). One-tenth of this cur- 
rent is taken as the working unit and called the ampere. 

The unit electrical resistance (one ohm) is that through 
which an electromotive force of one volt will force a current of 
one ampere. 

The C. G. S. unit of work is that due to unit force acting 
through unit distance; that is, one dyne acting through one 
centimetre. As this is too small to be generally convenient, 
ten million times this amount is taken as the working unit 
(called the joule). This is a little less than three-quarters of a 
foot-pound (exactly .7373). The unit rate of doing work is 
one joule per second. This unit rate is called the watt, and 
translating this into English measure, one watt equals 74V 
horse-power. 

Although the watt is often spoken of as an electrical unit, it 
belongs no more to electrical than to any other form of energy. 
It only remains to show the relation of the watt to the more 
strictly electrical units just mentioned. Recurring to our 
definition of the volt, let us suppose that the resistance of the 
circuit of which the moving wire is a part is such that unit 
electromotive force produces unit current in it. The stress 
between the field of the moving wire and the other unit field 
in which it moves is then one dyne at unit distance. In main- 
taining this for one second at the given rate of moving (1 cm. 
per second) the work done is, as above, one C. G. S. unit. At 
this rate, if the E. M. F. were 1 volt and the current 1 ampere, 
the work would be one joule and the rate of doing work one 
watt. If either E. M. F. or current were changed, the work 
would be proportionally changed. So, the number of volts 
multiplied by the number of amperes is numerically equal to 
the watts, i.e., we have obtained the dynamical equivalent 
of the two factors that make up electrical energy as ordinarily 
reckoned. So the output of any dynamo in watts is deter- 
mined by the volt-amperes produced, and we see the reason 
of the ordinary statement that 746 volt-amperes make one 
horse-power. This is always' true whether the output is steady 



22 ELECTRIC TRANSMISSION OF POWER. 

or variable, so long as we give to the product of volts and 
amperes their true concurrent values. 

What few other electrical units appear in practical work will 
be referred to in their proper places. 

It has been the purpose of this chapter, not so much to set 
forth the ordinary elements of electrical study, as to present 
these elements as viewed from the standpoint of energy. The 
author has purposely avoided the conception of electricity as 
a material something, to lay the greater emphasis on the 
paramount importance of electrical energy. The present 
recrudescence of a material theory of electrical charge in no 
way affects the validity of the principles here laid down, since 
it deals merely with a possible mechanism behind the stresses 
and strains which are experimentally apparent. 



CHAPTER II. 

GENERAL CONDITIONS OF POWER TRANSMISSION. 

The growth of human industry depends on nothing more 
than upon the possession of cheap and convenient power. 
Labor is by far the largest factor in the cost of many manu- 
factured articles, and in so far as motive power is cheap and 
easy of application it tends to displace the strength of human 
hands in all manufacturing processes, and so to reduce the 
labor cost and to set free that labor for other and less purely 
machine-like purposes. 

Therefore industrial operations have steadily gravitated 
toward regions where power is easily procured, often at the 
sacrifice of certain other advantages. This is in no wise 
better shown than by the growth of cities around easily avail- 
able water-powers, even in regions where both raw material 
and finished product became subject to considerable cost of 
transportation. With the introduction of the steam engine 
came a corresponding tendency to gather factories about 
regions of cheap fuel. These, localities, like those in which 
water-power is plentiful, seldom coincide with centres of cheap 
material and transportation, so that it has generally been 
desirable to strike an average condition of maximum economy 
by transporting the necessary power, stored in the form of 
fuel, to some advantageous point. 

Experience has shown, however, that, while the hauling of 
coal is a simple and comparatively cheap expedient, fuel 
utilized for running heat engines is in very many cases so 
much more expensive than hydraulic power as to be quite out 
of competition in cases where the latter can be transmitted, 
with a reasonable degree of economy, to places that are favor- 
able for its utilization. And in general it is found that there 
is a wide field for the transmission of power obtained from a 
given source, in competition with power from some other 
source utilized in situ. 

23 



24 ELECTRIC TRANSMISSION OF POWER. 

The sources of energy on which we may draw for mechanical 
power to be employed on the spot or transmitted elsewhere 
are very diversified, although few of them are to-day utilized 
in any considerable amount. Taking them in the order of 
their present importance we arrive at something like the fol- 
lowing classification: 
I. Fuel. 
II. Water-power. 

III. Wind. 

IV. Solar radiation. 

V. Tidal and wave energy. 

VI. Internal energy of the earth. 

Of these only the first two play any important part in our 
industrial economy. The third is employed in a very small 
and spasmodic way, the fourth and fifth although enormous in 
amount are almost untouched, while the last is not at present 
used at all, owing to inherent difficulties. 

I. The world's supply of fuel is almost too great for intel- 
ligible description. Aside from a widely distributed and 
steadily renewed supply of wood, the extent and capacity of 
available coalfields give promise that for a very long time to 
come fuel will be the chief source of energy. Coal is found 
in nearly every country, and in most quite plentifully, while 
exploration both in old fields and in new, is constantly bringing 
to light fresh supplies. Many computations concerning the 
probable duration of the coal supply have been made, but they 
are generally unreliable owing to the great probability that 
only a very small proportion of the available coal is as yet 
known to mankind. Certain it is that there is unlikely to be 
a marked scarcity of fuel for several centuries to come, even at 
the present rate of increase in its consumption. Still, it is 
altogether probable that it may become considerably dearer 
than at present within perhaps the present century, owing to 
the increased difficulty of working the older mines and the 
comparative inaccessibility of new ones. 

Besides coal we have petroleum and natural gas in unknown 
but surely very great quantities, since the distribution of both 
is far wider than has generally been supposed. At present the 
cost of these as fuel does not differ widely from that of coal, 



GENERAL CONDITIONS OF POWER TRANSMISSION. 25 

but appearances indicate that they are likely to be sooner 
exhausted. 

Every improvement that is made in the generation of power 
by steam and its subsequent distribution, helps to economize 
the fuel supply and stave off the already distant day when fuel 
shall be scarce. The work of the past half century has by direct 
improvement in steam practice, nearly if not quite doubled 
the energy available per ton of fuel. Beyond this much has 
been done along collateral lines. Particularly, explosive vapor 
engines have been developed to a point at which they are for 
small powers decidedly more economical than steam engines. 
Gas engines of moderate size, 5 to 25 HP, are readily ob- 
tained of such excellence as to give a brake-horse-power hour 
on an expenditure of little, if any, more than 20 cu. ft. of ordi- 
nary gas, reducing the cost per HPH to below that of power 
from a steam engine of similar size. Engines using an explo- 
sive mixture of air and petroleum vapor are at least equally 
economical, in fact more so unless the comparison be made 
with very cheap gas. 

These explosive engines have nearly double the net efficiency 
of steam engines as converters of thermal energy into mechani- 
cal power, and are capable of giving under favorable circum- 
stances 1 HPH on the thermal equivalent of less than 1 pound 
of coal. 

II. Water-power derived from streams is not distributed with 
the same lavish impartiality as fuel, but nevertheless exists in 
many regions in sufficient amount to be of the greatest impor- 
tance in industrial operations. Available streams exist around 
almost every mountain range and are capable of furnishing an 
amount of power that is seldom realized. In the United States 
the total horse-power of the improved water-power is approxi- 
mately 1,500,000. New England is especially rich in this re- 
spect, as is, too, the entire region bordering on the Appalachian 
range. The Rocky Mountains are less favored, the available 
water being rather small in amount, on account of the smaller 
rainfall and the severe cold of the winters. 

The Pacific slope is rather better off, and the high price of coal 
operates to hasten the development of every practicable power. 
All over the country are scattered small water-powers, and one 



26 ELECTRIC TRANSMISSION OF POWER. 

of the interesting results of the growth of electrical power 
transmission has been to bring to light half forgotten falls, even 
in familiar streams. Abroad, Switzerland is rich in powers of 
moderate size, as is the entire Alpine region, while a few years 
of experience in electrical transmission will probably cause the 
discovery or utilization of many water-powers that have hardly 
been considered, even in highly developed countries. Of the 
world's total water-power supply we know little more than 
of its coal supply, but it is quite certain, now that transmission 
of power over very considerable distances is practicable, that 
the employment of the one will every year lessen the relative 
inroads upon the other. And this is in spite of the fact that 
water is by no means always cheaper than steam as a motive 
agent. 

III. Wind as a prime mover has been employed on a rather 
small scale from the very earliest times. Were it not for the 
extreme irregularity of the power supplied by it in most places, 
the windmill would be to-day a very important factor in the 
problem of cheap power. Unhappily, winds in the same place 
vary most erratically, from the merest breeze to a hurricane 
sweeping along at the rate of 50 to 75 miles an hour. As all 
strengths of wind within very wide limits must be utilized by the 
same apparatus running at all sorts of speeds, it is no easy 
matter to employ it for most sorts of work. It seems especially 
unfitted for electrical work, and yet several small private plants 
have obtained good results from windmills used in connection 
with storage batteries. 

In ordinary winds the great size of the wheel necessary for a 
moderate power militates against any very extensive use. For 
example, with a good breeze of 10 miles per hour a wheel about 
twenty-five feet in diameter is needed to produce steadily a 
single effective horse-power, and the rate of rotation, about 30 
revolutions per minute, is so low as to be inconvenient for many 
purposes. Hence windmills are generally used for very small 
work which can be done at variable speed, such as pumping, 
grinding, and the like, for which they are unexcelled in cheap- 
ness and convenience. For large work we can hardly count 
much on wind-power, in spite of ingenious speculations to the 
contrary, and as a source of power for general distribution it 



GENERAL CONDITIONS OP POWER TRANSMISSION. 21 

is out of the question, for such as it is we have it already dis- 
tributed. It must rather be regarded as a local competitor of 
distributed power, and even so only in a small and limited field. 
IV. Aside from being in a general way the ultimate source 
of nearly all terrestrial energy, the sun steadily furnishes an 
amount of radiant energy, which if converted into mechanical 
power would more than supply all possible human needs. Its 
full value is the equivalent of no less than ten thousand horse- 
power per acre of surface exposed to the perpendicular rays of 
the sun. 

This prodigious amount is reduced by perhaps one-third 
through atmospheric absorption before it reaches the sea level, 
and in cloudy weather by a very much larger amount. Never- 
theless, with clear sunlight the amount of energy practically 
available, after making all allowances for increased absorption 
when the sim is low, and for the hours of darkness in any given 
place, is very great. If we suppose the radiant energy to be 
received on concave mirrors kept turned toward the sun and 
arranged so as to utilize the heat in the boiler of a steam or 
vapor engine, the average result after making all allowances for 
losses would be one mechanical horse-power for each 100 square 
feet of mirror-aperture, available about ten hours per day. 

Very important pioneer work was done on solar engines 
by John Ericsson and by M. Mouchot more than a quarter 
century ago, but it is only within the past few years that the 
solar engine has approached really commercial form. At the 
present time solar heating apparatus is being regularly pro- 
duced although on a rather smaU scale, and gives good economic 
results. The solar motor is essentially a steam engine supplied 
with steam by a boiler placed in the focus of a concave mirror. 
This is shaped like an open umbrella with its handle pointed 
toward the sun. The umbrella is carried on a polar axis at 
right angles to the handle and pointing toward the celestial 
pole. The actual mirror is segmental, built upon a steel 
frame, of rectangles of plane thin glass silvered on the back. 
Each segment is about six inches wide and two feet long, sup- 
ported by cushioned clamps at the corners, and the whole are 
arranged to focus the sun's rays on a cylindrically disposed, 
blackened boiler formed of copper tube. The structure is 



28 ELECTRIC TRANSMISSION OF POWER. 

supported on a polar axis about which it is moved automatically 
by steps every few minutes, remaining locked in the intervals 
to avoid needless strain on the clockwork. There is also a 
motion in declination to take care of the apparent motion of 
the sun, adjusted by hand every day or two as becomes neces- 
sary. The engine is generally a rather highly organized one, 
worked condensing and with superheated steam at a pressure 
of 200 lbs. per square inch or more. The net result is one brake 
HP for each 100 square feet of mirror surface. The mirror 
structure becomes rather unwieldy when of dimensions great 
enough to supply an engine of rnore than 15 HP, so that for 
greater powers several mirrors with their boilers should be 
coupled together. The initial cost of each equipment is high, 
say $250 per horse-power, but the fuel cost is nil and the 
attendance required very little, so that even now there are 
localities where its use is economical. The full power is 
available about eight hours per day, and there is upon the earth's 
surface a vast, irregular equatorial belt in which such solar 
engines can be successfully used for irrigation and other pur- 
poses. The power is steady, and reliable during the hours of 
sunshine, and gives constant speed like any other steam engine. 
It is worth mentioning that general heating and cooking appa- 
ratus on the same plan is entirely practicable in regions of 
scant fuel and high sun, and has been tried successfully. 

V. Of tidal energy but little use has yet been made. Here 
and there, both here and abroad, are small tidemills, feebly sug- 
gesting the enormous store of tidal power as yet unutilized. 
The intermittent character of tidal currents and the small 
extent of the rise and fall generally available, make the practical 
part of the problem somewhat difficult. The easiest way of 
harnessing the tides is to let the rising water store itself in 
artificial reservoirs, or natural ones artificially improved, and 
then during the ebb to use it with water-wheels. But usually 
the head is so small that for any considerable power stored the 
area of reservoir must be very large, and the wheels must be 
of great size in order to make the stored water do its work 
before the rising tide checks further operations. The average 
tide is seldom more than 10 to 12 feet along our coast, and of 
this hardly more than half could be utiUzed to give even a few 



GENERAL CONDITIONS OF POWER TRANSMISSION. 29 

hours of daily service. At 6 feet available head about 100 
cubic feet of water must be stored for each horse-power-minute, 
even with the best modern turbines. Hence for say 1,000 
HP available for 5 hours there must be impounded 30,000,000 
cubic feet of water, making a pond 6 feet deep and almost 
120 acres in extent. 

Tidal operations are therefore likely to be restricted to a 
few favored localities where through special configuration of 
the ground natural reservoirs can be found, and where the rise 
of the tide is several times the figure named. In rare cases, by 
the use of more than one reservoir and outlet, work may be 
made nearly or quite continuous. Still, with all these difficul- 
ties the possibilities of tidal power are enormous in certain 
cases. Take for example the Bay of Fundy with its 40 feet of 
normal tidal rise. If half this head can be used in practice 30 
cubic feet will be required per horse-power-minute, and a single 
square mile of reservoir capacity gained by damming an estu- 
ary or cutting into a favorable location on shore will yield 
62,000 horse-power ten hours per day in two five-hour intervals. 
Generally speaking, economic conditions are not favorable for 
such an employment of the tides, but in some localities a 
peculiarly fortunate contour of the shore coupled with high 
local cost of fuel may render it easy and profitable to press the 
tides into service. The author has had occasion to investigate 
a few cases of this kind in which the commercial outlook was 
good. The main difficulties in utilizing the tides are two : first, 
the very variable head; and, second, the short daily periods in 
which the outflow can be advantageously used. Moreover, 
these periods shift just as the times of high tide shift, by a 
little less than an hour per day, so that if the power were used 
directly it would often be available only at very inconvenient 
times. 

To work the tides on a really commercial scale, therefore, 
some system of storing power is absolutely necessary. And 
since one would have to deal with very large amounts of 
power, much of the time the entire output of the plant, the 
storage must be fairly cheap and efficient. For work on the 
scale contemplated, it is probable that the storage battery is 
the most available method. Used in very large units in 



30 ELECTRIC TRANSMISSION OF POWER. 

a colossal plant, most of the serious objections to the 
storage battery are in great measure obviated, since attend- 
ance and repairs can be part of the duties of a regular main- 
tenance department, inspecting, testing, and repairing damaged 
cells, casting and filling new plates, and keeping the plant in 
first-class working condition all the time. 

The cost of battery would be, of course, a serious matter, but 
not prohibitive, and its efficiency could probably be kept as 
high as 80 per cent. The best idea of the economic side of 
the case can be gained by investigating a hypothetical case of 
tidal storage, based, for convenience, on the square mile 
of reservoir just mentioned. To simplify the case we will 
assume use of the power locally, so as not to complicate the 
situation by the details of a long-distance transmission. We 
will take the generators, which can be worked at steady full 
load, at 94 per cent efficiency. Then the efficiency to the dis- 
tributing lines would be 

.94 X .80 = .752. 

At this rate, the 62,000 HP available would give substan- 
tially 35,000 KW; i.e., 350,000 KW-hours daily. Storage 
capacity would have to be provided for this whole amount 
in a gigantic battery, weighing about 18,000 tons and cost- 
ing in the neighborhood of three milHon dollars. To this, 
of course, the cost of the electrical and hydraulic machinery 
must be added, and beyond this must be reckoned the really 
very uncertain cost of the reservoir and hydraulic work. In 
spite of all this, an assured market for the output would lead 
to economic success under conditions quite possible to be 
realized. If extensive transmission had to accompany the 
enterprise there would be still further loss of efficiency, so 
that the final figure would not exceed 60 per cent, which 
would reduce the salable power to about 27,000 KW. Evi- 
dently this would have to command a very good price, to carry 
the burden of the heavy investment, which would probably 
rise to between $10,000,000 and $15,000,000. The cost of 
such an enterprise is so formidable that it is practically out of 
the question, unless it can reach a market for power in which 
a very high price is admissible. When fuel begins to get 



GENERAL CONDITIONS OF POWER TRANSMISSION. 31 

scarce it will be profitable to utilize the tides on a large scale: 
until then, their use will be confined to isolated cases in which 
local causes lead to high cost of other power and tidal storage 
is unusually cheap. 

All these considerations apply with similar force to wave 
motors, which have been often suggested, and now and then 
used, as sources of power. The energy of the waves is very 
great, as the havoc wrought by storms bears witness; but it is 
most irregular in amount, and requires very large apparatus 
for its utilization. What is worse, the power is intermittent, 
so that to be of any material advantage it must be brought to 
a steady output by means of storage of energy in some form. 
The periodicity of wave motion is so low, roughly about 6 to 
10 crests per minute, that flywheels and the like are of little use, 
and storage is practically reduced to a question of compressing 
air or pumping water. Even if some such wasteful intermedi- 
ary were not necessary, and one could work directl}^ by means 
of floats or their equivalent, a float would have to have a dis- 
placement of at least one ton per horse-power, even if work- 
ing in a pretty heavy sea, and under ordinary circumstances 
several times that amount of displacement. At best, wave 
motors are cumbersome, and give small promise of economic 
development while other sources of energy are available. 

VI. Of the earth's internal heat energy there is little to be 
said. It is quite unused save as an occasional source of hot 
water, and except in a very few cases could not be employed at 
all, much less to any advantage. Immense as is its aggregate 
amount, it is, save at isolated points, so far separated from the 
earth's surface as to be very difficult to get at. Hot springs, 
very deep artesian wells, and some volcanic regions, furnish the 
only feasible sources of terrestrial heat energy, so that the 
whole matter is only of theoretical interest. 

We see that at present only two sources of energy, viz., 
fuel and water-power, are worthy of serious consideration in 
connection with the general problem of the transmission and 
distribution of power. The other sources enumerated are 
either very irregular, uncertain in amount, or so difficult of 
utilization as to remove them at once from the sphere of prac- 
tical work. 



32 ELECTRIC TRANSMISSION OF POWER. 

Granted, then, that fuel and water-power are and are hkely 
long to remain the dominant sources of energy, let us look more 
closely into their possibilities. From each energy can be 
readily transmitted and distributed by any suitable means; 
each, in fact, can be transferred bodily to a distant scene of 
action without any transformation from its own proper form. 
In fact, for certain purposes and under certain conditions such 
is the very best method. Fuel for ordinary heating and water 
for such uses as hydraulic mining can be taken as cases in 
point. In a more general way, both fuel and water for the 
development of mechanical power may often profitably be 
transferred from place to place. 

The conditions of economy in the transmission of fuel as 
such are comparatively easy to examine and define. Coal 
may be produced at the mine for a certain quite definite 
cost per ton. It can be transported over railroads and 
waterways for an easily ascertainable price. Such a trans- 
mission may be said to have a definite efficiency, as for example 
90 per cent, when the total transportation charges against a ton 
of coal amount to 10 per cent of its final value. From this 
standpoint it is quite possible to transmit power at this very 
high efficiency even to the distance of hundreds of miles. If 
the final object be the distribution of power on a large scale, as 
from a great central station, this transmission by transporta- 
tion of fuel is often at once the most reliable and the cheapest 
method. 

Transformation of the fuel energy at its source into some 
other form for the purpose of transmission is generally only 
justifiable, first, when by so doing fuel not available for trans- 
portation at a high efficiency can be rendered valuable by trans- 
formation of its energy, or second, when it is to be utilized 
at some distant point in a manner which compels a loss of 
efficiency greater than that encountered in transmission. As 
an example of the first condition, fully one-third of the coal as 
ordinarily mined is unfitted, through its finely divided condition 
or poor quality, for transportation over considerable distances. 
Its commercial value is so small per ton that it could not be 
carried far without incurring charges for carriage amounting 
to a large part of its value. Hence, every coal mine accumu- 



GENERAL CONDITIONS OF POWER TRANSAIISSION. 33 

lates a mountainous culm pile that is at present not only 
valueless but cumbers the ground. This waste product could 
sometimes be very profitably employed in generating power 
which could be transmitted at a relatively ver}^ high efficiency 
and sold at a good price. 

A specimen of the second kind may be found in the somewhat 
rare case of power which must be used in small units scattered 
over a considerable territory, so that they could be replaced 
with a great gain in efficiency by a single large generating 
station. Such a state of affairs might be found in certain 
mining regions where coal and iron mines are interspersed. 
This must not be confounded with the very ordinary case of 
distributing energy from a central station to various scattered 
points, for we are here considering only the original source of 
the fuel. 

When an extensive distribution of energy from a power 
station is contemplated, electrical or similar transmission of 
power to that station is generally economical only on the condi- 
tion above expressed, of using fuel otherwise valueless, since the 
facilities for transportation to points at which power distribu- 
tion on a large scale would be profitable, are generally good 
and fairly cheap. All this applies to piping gas or petroleum 
as well as to hauling coal, with the difference that neither gas 
nor petroleum has any waste corresponding to culm, and hence 
the transportation of each of them becomes a process entirely 
comparable with the transmission of energy and directly com- 
peting therewith. It has even been proposed to pipe coal dust 
by pneumatic power for fuel purposes. 

Water-power is by no means always cheaper than fuel, but as 
a general rule it is, and by such an amount that it can be 
transformed into electrical energy and transmitted to at least 
a moderate distance without losing its economic advantage. 
It therefore is usually the cheapest source from which to derive 
power for general distribution on a large scale. 

It is very difficult to give a clear idea of the relative cost of 
steam and water-power, for while the one can be predicted for 
any given place with fair accuracy, the other is subject to 
immense variations. Once established, a water-power plant 
can be operated very cheaply, but the cost of developing the 



34 ELECTRIC TRANSMISSION OF POWER. 

water-power may be almost anything, and each case must be 
figured by itself. It is easy to obtain estimates of the cost of 
developing a given stream and to form a close estimate of 
both the interest charges to be incurred and the additional 
expense of repairs and of operation. The cost of steam- 
power for the same conditions can be accurately estimated. 
The details of such estimates we will discuss later. In general, 
one can only safely say that the costs of steam and water- 
power overlap, as it were, so that while the more easily 
developed water-powers are cheaper sources of energy than 
fuel at any ordinary price, there are many cases in which the 
great cost of development of difficult water-powers prohibits 
competition with steam except where fuel is very dear. Much 
depends on the topography of the country, the amount and 
reliability of the available head of water, the price at which 
water rights can be obtained and various other local conditions. 
To utilize the normal minimum power of a stream is gener- 
ally comparatively easy, while so to take account of high water 
as to obtain nearly the full continuous working power of the 
stream often means great added expense for storage capacity 
and works to control and regulate the flow. 

In addition we have to consider two distinct phases of the 
comparative cost — first, the cost of steam and water as prime 
movers for a source of power to be distributed, and second, 
the relation between these costs and that of steam-power at 
the points where the distribution takes place. 

Given a proper source of energy, there is vast variety in the 
character of the work of transmission and distribution that is 
to be undertaken. In the first place, the point of utilization 
may be distant anywhere from a few hundred feet to many 
miles, and at that point the object may be the delivery of 
mechanical power in a single unit, in one or several groups of 
allied units, in one or several widely scattered groups, or 
finally for transformation into some other form of energy in 
the most direct way possible. 

There is no single method of power transmission which 
meets in the best possible manner all these widely varying con- 
ditions. Although electrical transmission is the most genera] 
solution of the difficult problem in hand, there are cases in 



GENERAL CONDITIONS OF POWER TRANSMISSION. 35 

which other methods are preferable and should be adopted. 
Those besides electric transmission which have come into con- 
siderable use are the following: 
I. Wire Rope Transmission. 
11. Hydraulic Transmission. 

III. Compressed Air Transmission. 

IV. Gas Transmission. 

It will be well to look into the distinguishing characteristics 
of these and their relation to electrical transmission, with the 
purpose of finding the advantages and limitations of each, so 
that the proper economic sphere of each may be determined, 
before taking up the electrical work which forms the main 
subject of this volume. Each method will be found to have 
its own legitimate place. 

I. The transmission of power by wire ropes is merely a very 
useful extension of the ordinary process of belting. Belts are 
made of material which will not stand exposure to the weather, 
and which being of low tensile strength is heavy and bulky in 
proportion to the power transmitted. The advantage of wire 
rope over belting lies in its high tensile strength and freedom 
from deterioration when used out of doors. To gain the 
fullest benefit from these properties it is necessary to use 
light ropes driven at high speed. 

It should be borne in mind that the power transmitted by 
anything of the nature of belting depends directly on the 
speed and the amount of pull exercised. If the force of the 
pull is 100 pounds weight and the speed of belt or rope is 
4,000 feet per minute, the amount of power transmitted is 
400,000 foot-pounds per minute or (since 1 horse-power is 33,000 
foot-pounds per minute) about 12 HP. The greater the speed, 
the more power transmitted with the same pull, or the less 
the pull for the same power. Wire rope can be safely run 
at a considerably higher speed than belting and is much stronger 
in proportion to its size and weight. It does not often replace 
belting for ordinary work, for the reason that owing to its 
small size it does not grip ordinary pulleys anywhere nearly 
in proportion to its strength. Hence, to best take advantage 
of its ability to transmit large powers, the rope speed must be 
high and the pulleys unusually large in diameter to give sufR- 



36 ELECTRIC TRANSMISSION OF POWER. 

cient surface of contact. Such large wheels are inconvenient 
in most situations, and as the alternative is a number of ropes 
which are troublesome to care for, rope driving save for outdoor 
work is rather uncommon. 

A typical rope transmission is shown diagrammatically in 
Fig. 10. Here A and B are two wheels, usually of cast iron, 
generally from 5 to 15 feet in diameter and with deeply grooved 





Fig. 10. 



rims. They are connected by a wire rope perhaps from ^ inch 
to IJ inch in diameter, which serves to transmit the power as 
the wheels revolve. The rope speed is usually from 3,000 to 
5,000 feet per minute, sometimes as high as 6,000. The distance 
between the centres of A and B may be anything required by 
the conditions up to four or even five hundred feet. Greater 
distances are seldom attempted in a single span, as, if the rope 
is not to be overstrained by its own weight, it must be allowed 
to sag considerably, compelhng the pulleys to be raised to 
keep it clear of the ground, and subjecting it to danger from 
swaying seriously by reason of wind pressure or other acci- 
dental causes. 

The rope employed is of special character. The material is 
the best charcoal iron or low steel, and the strands are usually 




Fig. 11. 



laid around a hemp core to give added flexibility. The rope 
generally employed in this country is of six strands with seven 
wires per strand, and is shown in cross-section in Fig. 11. 
Even with the hemp core there is still in an iron rope sufficient 
resistance to bending to make the use of pulleys of large 



GENERAL CONDITIONS OF POWER TRANSMISSION. 37 



diameter necessary. Sometimes each separate strand is made 
with a hemp core, or is composed of nineteen small wires 
instead of seven larger ones, to increase the flexibility and to 
make it possible to use smaller sheaves and drums, as in hoist- 
ing machinery. 

Steel rope is slightly more costly than iron, but gives greater 
durability. The wheels on which these ropes run are fur- 
nished with a deep groove around the circumference, pro- 
vided with a relatively soft packing at the bottom on which 
the rope rests, and which serves to increase the grip of the 
rope and to decrease the wear upon it. Fig. 12 shows a section 
of the rim of such a wheel. The bushing at the bottom of the 




Fig. 12. 



groove, upon which the rope directly bears, has been made of 
various materials, but at present, leather and especially prepared 
rubber are in most general use. The small pieces of which 
the bushing is composed are cut to shape and driven into the 
dovetailed recess at the bottom of the groove. The bushings 
have to be replaced at frequent intervals, and the cables them- 
selves have an average life of not much over a year. 

When a straightaway transmission of a few hundred feet is 
necessary, when the power concerned is not great, and the size 
of the pulleys is not a serious inconvenience, this transmission 
by wire rope is both very cheap and enormously efficient. 
No other known method can compete with it within these 
somewhat narrow limitations. For a span of ordinary length 



38 ELECTRIC TRANSMISSION OF POWER. 

and the usual rope speeds, the efficiency has been shown by 
experiment to be between 96 and 97 per cent. At a dis- 
tance of four to five hundred feet the weight and sag of the 
rope becomes a very serious inconvenience, and the arrange- 
ment has to be modified. Perhaps the most obvious plan is 




Fig. 13. 

to introduce a sheave to support the slack of the cable, as 
shown in Fig. 13. 

On longer spans several sheaves become necessary, and both 
the slack and the tight portions of the cable need such support. 
In cable railway work, the most familiar instance of power 
transmission by wire ropes, numerous sheaves have to be 
employed to keep the cable in its working position in the 
somewhat contracted conduit. These reduce the efficiency of 
the system considerably, so that the power taken to run the 
cable light is often greater than the net power transmitted. 
In aerial cable lines multiple sheaves are seldom used, and the 
more usual procedure is to subdivide the transmission into 
several independent spans, thus lessening swaying and sagging 
as well as the length of rope that must be discarded in case of a 
serious break. This device is shown in Fig. 14. It employs 
intermediate pulley stations at which are installed double 






Fig. 14. 



grooved pulleys to accommodate the separate cables that 
form the individual spans. Such a pulley is shown in section 
in Fig. 15. The spans may be three or four hundred feet 
long; as soon as the length gets troublesome another pulley 
station is employed. There is necessarily a certain small loss of 
energy at each such station. This is approximately proportional 
to the number of times the rope passes over a pulley. From 



GENERAL CONDITIONS OF POWER TRANSMISSION. 89 

the best experimental data available the efficiency of a rope 
transmission extended by separate spans is nearly as follows: 



Number of spans . . . 
Per cent efficiency . . 



. 1 2 3 

.96 .94 .93 



4 
.91 



6 7 8 9 10 
.87 .86 .85 .84 .82 



These figures are taken to the nearest per cent and are for 
full load only. At half-load the loss in each case would be 
doubled. For instance, a 10-span transmission at half-load 




Fig. 15. 



would give about 64 per cent efficiency. The pulley stations 
consist of the double-grooved wheel before mentioned mounted 
on a substantial and rather high pedestal or frame-work. 
In this country a timber frame is generally used; abroad 




a masonry pier is more common. A convenient form of 
frame- work is shown in Fig. 16. An idea of its dimensions may 
be gained from the fact that the wheel is likely to be 6 to 10 
feet in diameter. 

It is interesting to note that the efficiency just given for a 10- 



40 



ELECTRIC TRANSMISSION OF POWER. 



span transmission at full load is quite nearly the same as would 
be obtained from an electrical-power transmission at moderate 
voltage over the same distance, assuming a unit of say 50 HP 
or upward. The first cost of the latter would be considerably 
higher than that of the rope transmission, but the repairs 
would certainly be much less than the replacements of cable, 
bringing the cost per HP at full load to about the same fig- 
ure by the two methods. 

From actual tests of electrical apparatus we have the fol- 
lowing efficiency for a transmission of 50 HP 5,000 feet, assum- 
ing 2,000 volts and 2 per cent line loss, which would require 



roMT 



S 



Fig. 17. 



a wire less than one-fourth of an inch in diameter.' Efficiency 
at full load 81 per cent, at half-load 72. These values are 
lower than those attainable with machinery of the most recent 
type, which should give at least 86 per cent at full load and 
80 per cent at half-load for the complete transmission, which 
beats out rope transmission at a distance much less than 
5,000 feet. Except at full load the electrical transmission has 
a very material advantage. This advantage would be greatly 
increased if the transmission were in anything but a straight 
Hne. An electric hne can be carried around any number of 
corners without loss of efficiency, while a rope transmission 
cannot. If it becomes needful to change the direction of a 



GENERAL CONDITIONS OF POWER TRANSMISSION. 41 

rope drive, it is done at a station provided with a pair of rope 
wheels connected by bevel gears set at any required angle. 
Fig. 17 shows such a station in diagram. The loss of energy 
in such a pair of bevel gears amounts to from 7 to 10 per 
cent, more often the latter. The bevel gears may be avoided 
by a sheave revolving in a horizontal plane, and carrying 
the turn in the cable, but while this arrangement is tolerably 
efficient, it greatly decreases the life of the rope. 

From what has been said, it will be seen that while cable 
transmission is for short distances in a straight line both cheap 
and very efficient, at 2,000 to 3,000 feet it is equalled and sur- 
passed in efficiency by electric transmission, with lesser main- 
tenance although greater first cost. The steel rope for a 50 
HP transmission of 5,000 feet would cost about $400, and 
replacement brings a considerable charge against each HP 
delivered. If the transmission is not straightaway, .or if 
branches have to be taken off en route, the efficiency of the 
system is considerably reduced by gear stations, while even 
aside from these the efficiency is high only at or near full load. 
But the general simplicity and cheapness of cable transmission 
have made it a favorite method, and there have been many such 
installations, some of them of a quite elaborate character. 
Most of them are small, since the amount of power that can 
be transmitted by a single rope is limited to 250 or 300 HP. 
Ropes suited to a larger power are too heavy and inflexible; 
1^ inch is about the greatest practicable diameter of cable, 
and even this requires pulleys between 15 and 20 feet in diam- 
eter for its proper operation. Besides, even at moderate 
distances the rope transmission suffers in wet or icy weather, 
so that at anywhere nearly equal costs the electrical drive 
is to be preferred save in the simplest cases. 

Under all circumstances the need of replacing the cables 
every year or so causes a high rate of maintenance. The fol- 
lowing table, giving the sizes of iron-wire cables and pulleys 
necessary for transmitting various amounts of power, will help 
to give a clearer idea of the conditions of cable transmission 
and aid in defining its limited but useful sphere. Speed is given 
in revolutions per minute, and pulley diameter is the smallest 
permissible. These figures are, as will readily be seen, for rope 



42 



ELECTRIC TRANSMISSION OF POWER. 



Diameter of Rope. 


Speed. 


Diameter of Pulley. 


HP. 


I'' 


150 


6' 


25 


9 '' 


140 


r 


35 


1: 


140 


%' 


45 


100 


10' 


85 


^?// 


80 


12' 


100 


\" 


80 


14' 


140 


ir 


80 


14' 


150 



speeds of not far from 3,000 feet per minute. This can fre- 
quently be safely raised to 5,000 with somewhat larger pulleys 
than those given and increased revolutions, while for steady 
loads the tension can be sHghtly augmented without danger. 
So while the figures given are those suitable for ordinary 
running with a good margin of capacity, the HP given can be 
nearly doubled when all conditions are favorable. 

But from all these figures it is sufficiently evident that 
rope transmission is very Umited in its apphcabihty and is 
not at all suited to work of distribution in small units. For 
a good many years, however, a wire-rope transmission, now 
practically superseded by electric driving, was operated at 
Schaffhausen on the falls of the Rhine. The power station 
dehvered more than 600 HP to a score of consumers over 
distances of half a mile or so. There were two bevel-gear 
stations, and on the average, five cable spans between the 
power station and the consumer, so that the efficiency even 
at full load was somewhere between 60 and 70 per cent and 
ordinarily very m.uch less. Nevertheless, in default of any 
better means of transmission at the time of installation, some 
twenty years since, the plant did fairly successful work, even 
from a commercial standpoint. In this country the system 
is very little used save for short straight runs between building 
and building across streets, for instance. 

II. Noting, then, that cable transmission does excellent work 
in its proper place, but is unsuited for the distribution of power 
or for transmissions of anything save the simplest sorts, we 
may pass to the hydrauhc method of transmitting and distrib- 
uting power. This in its crude form of small water-motors 
attached to ordinary city mains is very famihar, but nothing 



GENERAL CONDITIONS OF POWER TRANSMISSION. 48 

more extensive has been attempted in this country. Abroad 
there are a number of hydraulic power plants specially intended 
for the distribution of power for general use, and the method is 
one which has been fairly successful. There are two distinct 
types of hydraulic plant, one utilizing such pressure as is 
available naturally or by pumping to reservoirs, the other 
employing very high artificial pressures, up to 750 pounds per 
square inch, and used only for special purposes. 

There are somewhat extensive works of the former kind at 
Zurich, Geneva, and Genoa, the effective head of water being 
in each case not far from 500 feet. In each case the power 
business has been an outgrowth of the municipal water-supply 
system. At Zurich and Geneva elevated reservoirs are supplied 
by pumping stations driven by water-power. At Genoa the 
head is a natural one, 20 miles from the city, and much of the 
fall is utilized 18 miles from Genoa in driving the fine constant 
current electric plant described elsewhere in this volume. 

At Zurich there is in addition to the ordinary low pressure 
water system a special high service reservoir supplying power 
to a large electric station and to small consumers. Water is 
pumped 6,000 feet into this reservoir through an 18-inch main, 
and the total power service from both systems is something 
like 500 HP, reckoned on a ten-hour basis. The price charged 
is from $37 to $80 per HP per year. 

The Geneva plant is on a much larger scale, the total turbine 
capacity being about 4,500 HP. Here, as at Zurich, there are 
two sets of mains, one at nearly 200 feet head, the other at 
about 450. Each supplies water for both power and general 
purposes. The high pressure service reservoir is about 2^ 
miles from the city, and the working pressure is supplied 
indifferently from this or from the pumps direct. There is an 
electric light plant with 600 HP in turbines driven by the 
pressure water, and a large number of smaller consumers. 
Water is supplied to the electric light company for as low as 
$15 per HP per year. 

Both these installations are extensions of the city water 
service, and have done excellent work. Operated in this way 
the economic conditions are somewhat different from those to 
be found in a hydraulic plant established by private enterprise 



44 



ELECTRIC TRANSMISSION OF POWER. 



for power only. An inquiry into the efficiency of such a 
system may be fairly based on the facts given. At Zurich, for 
example, the efficiency from turbine shaft to reservoir cannot 
well exceed 75. The distributing mains must involve a loss 
of not less than 10 per cent, while the motors cannot be 
counted on for an efficiency of over .75. The total efficiency 
from turbine shaft to motor shaft is then about .75 X .75 X .90 
= 50.6 per cent. The character of the motors has an impor- 
tant influence on the economy of the system, particularly at 
low loads. The motors most used particularly for small powers 



r^ 




Fig. 18. 



are oscillating water engines of the type shown in Fig. 18. 
The form shown is made by Schmid of Zurich. It possesses, in 
common with all others of similar construction, the undesirable 
property of taking a uniform amount of water at uniform speed, 
quite irrespective of load. The mechanical efficiency falls off 
like that of a steam engine, friction being nearly constant. 
Better average results are secured with impulse turbines (see 
Chapter IX) of which the efficiency varies but little as the load 
falls off, or for high rotative speeds with impulse wheels like the 
Pel ton, shown in Fig. 19, as adapted for motors of moderate 
power. At half -load, i.e., half flow, the losses in distributing 



GENERAL CONDITIONS OF POWER TRANSMISSION. 45 

mains would be reduced to about one-third, while the efficiency 
of the engine motors would certainly not be lowered by less 
than 5 per cent. The total half-load efficiency would then be 
.75 X .97 X .70 = 50.9 per cent, actually a trifle higher than 
at full load. This apparently remarkable property is shared 
by all transmissions wherein the transmission loss proper is 
fairly large. 

The second type of hydraulic distribution of power is that 
at very high pressures and employing a purely artificial head. 
The pressures involved are usually 700 to 800 pounds per square 




Ftg. 19. 



inch, and a small amount of storage capacity is gained by 
employing what are known as hydraulic accumulators, fed by 
the pressure pumps. These accumulators are merely long 
vertical cylinders adapted to withstand the working pressure, 
which is kept up by a closely fitting and enormously heavy 
piston. The distribution of power is by iron pipes leading to 
the various water motors. This high pressure water system 
is a device almost peculiar to England, and has been slow in 
making headway elsewhere. Its especial advantage is in con- 
nection with an exceedingly intermittent load, such as is 
obtained from cranes, hoists, and the like. This is for the 
reason that with a low average output a comparatively small 



46 ELECTRIC TRANSMISSION OF POWER. 

engine and pump working continuously at nearly uniform load 
can keep the accumulators charged, while the rate of output 
of the accumulators is enormous in case of a brief demand for 
very great power. 

Power plants on this hydraulic accumulator system are in 
operation in the cities of London, Liverpool, Hull, and Bir- 
mingham, England, and at Marseilles, France. The Lon- 
don plant is the most important of those mentioned, con- 
sisting of three pumping and accumulator stations and about 
60 miles of mains. The total number of motors operated was 
in 1892 about 1,700. The charges are by meter, and are based 
on intermittent work, being quite prohibitive for continuous 
service — from $200 to $500 per effective HP per year of 3,000 
hours. The largest accumulators have pistons 20 inches in 
diameter and 23 feet stroke, giving a storage capacity of only 
24 horse-power-hours each. While very convenient for the 
supply of power for intermittent service only, this system, like 
hydraulic supply at low pressure, is rather inefficient, the more 
so as it has been found advisable to employ hydraulic motors 
of the piston type, although special Pelton motors have been 
used in some cases. 

Any hydraulic system suffers severely from the inefficiency 
of pump and motors and from loss of head in the pipes. The 
amount of power that can be transmitted in the mains is quite 
limited, since the permissible velocity is not large. About 
3 feet per second is customary — more than this involves 
excessive friction and danger from hydraulic shock. At this 
speed a pipe about 2 feet in diameter is necessary to transmit 
500 HP under 500 feet head.* The power delivered increases 
directly with the head, but as the pressure increases the largest 
practicable size of pipe decreases, and on the high pressure 
systems nothing larger than 12 inches has been attempted, and 
even this requires the use of solid drawn steel 

Whatever the size of pipe, the loss in head is quite nearly 
inversely as the diameter and directly as the square of the 
velocity. Even for high pressure systems this loss is by no 
means negligible, since the pipes used are rather small. 

The following table gives the loss of head in feet per 100 feet 
* Cost per mile laid in average unpaved ground about $15,000. 



GENERAL CONDITIONS OF POWER TRANSMISSION. 47 



of pipe and at a uniform velocity of 3 feet per second. This 
applies to pipe in good average condition. When the pipe is 
new and quite clean, the losses may be slightly less. If the 
pipe is old and incrusted, the above losses may be nearly 
doubled. Bends and branches still further reduce the working 
pressure. 



Diameter .... 


1" 


2'' 


3" 


4' 


5^' 


6" 


y. 


8'' 


10'' 


12'' 


Loss of Head . . . 


4.89 


2.44 


1.62 


1.22 


.98 


.81 


.70 


.61 


.49 


.41 



Diameter 



Loss of Head 



14" 


16" 


18" 


20" 


22" 


24" 
.20 


26" 
.19 


28" 
.17 


40" 
.16 


.35 


.32 


.27 


.25 


.22 



36' 



.13 



We may now look into the efficiency of these high pressure 
hydraulic systems. Of the mechanical horse-power applied to 
the pump we cannot reasonably hope to get more than 75 per 
cent as energy stored in the accumulators. Tests on the 
Marseilles plant have shown 70 to 80 per cent efficiency between 
the indicated steam power and the accumulators, the former 
figure at the speeds corresponding to full woi]king capacity. As 
the pumps were direct acting the difference between brake and 
indicated HP was presumably very small. The motors can be 
counted on for about .75 efficiency, and the losses of head in 
the pipes for any ordinary distribution cannot safely be taken 
at less than 5 per cent. Hence the full load efficiency is about 
.75 X .75 X .95 = .53. The efficiency at full load is thus not 
far from that of the low pressure system, but at half-load it 
suffers from the use of piston motors, generally necessary on 
account of the too high speed of rotary motors at high pressure. 
At even 500 pounds per square inch pressure the normal speed 
of a Pelton wheel of say 20 HP would be over 4,000 r. p. m., and 
could not be greatly reduced without seriously cutting down 
the efficiency. At half-load the piston motors could not be 
relied on for over .65 efficiency, reducing the total efficiency, 
even allowing for greatly lessened pipe loss, to about 45 per 
cent. On the whole, the hydraulic accumulator system must be 



48 ELECTRIC TRANSMISSION OF POWER. 

regarded as a very ingenious and occasionally useful freak. It 
may now and then be useful as an auxiliary in the storage of 
energy from a very irregular power supply. 

The strongest point of hydraulic transmission is its ready 
adaptability in connection with water supply systems for gen- 
eral purposes. Skilfully installed, as for instance at Geneva, 
it furnishes convenient, reliable, and fairly cheap motive power. 
As a distinct power enterprise the high first cost is against it, 
and the efficiency is never really good. All this applies with 
even greater force to the special high pressure systems, which 
suffer from inability to cope with continuous work, thus seri- 
ously limiting the possible market. Even for intermittent ser- 
vice the charges are enormously high. 

The methods of power transmission already mentioned are 
then somewhat limited in their usefulness by rather well 
defined conditions, which make their employment advisable in 
some cases and definitely inadvisable in general. 

III. We may now pass to the pneumatic method of transmit- 
ting power, which is far more general in its convenient appli- 
cability than either of the others, and which is the only system 
other than electric which has been extensively applied in prac- 
tice to the distribution of power in small units, although only 
short distances have been involved in any of the plants 
hitherto operated, and the possible performance at long dis- 
tances is more a subject of speculation than of reasonable cer- 
tainty. Transmission of power by compressed air involves 
essentially three elements: An air compressor delivering the 
air under a tension of from 50 to 100 or more pounds per 
square inch into a pipe system, which conveys the compressed 
air to the various motors. These motors are substantially 
steam engines in mechanical arrangements, and indeed almost 
any steam engine can be readily adapted for use with com- 
pressed air. The compressor itself is not unlike an ordinary 
steam pump in general arrangement. Its appearance in the 
smaller sizes is well shown in diagram in Fig. 20. The system 
was originally introduced about fifty years ago for mining 
purposes, and owed its early importance to its use in working 
the drills in the construction of the Hoosac, Mont Cenis, and 
St. Gothard tunnels. Since then it has come to be used on a 



GENERAL CONDITIONS OF POWER TRANSMISSION. 49 

very extensive scale for drilling operations, and recently has 
also been applied for the distribution of power for general pur- 
poses, particularly in Paris, where the only really extensive 
system of this kind is in operation. Its best field has been 
and still is in mining operations where the escaping air is a 
welcome addition to the means of ventilation and where, as a 
rule, the distances are not great. 

Transmission of power by piping compressed air has even 
for general distribution certain very well marked advan- 
tages. The subdivision of the power can be carried on to 




Fig. 20. 



almost any extent, and the motors are fairly efficient, simple, 
and relatively cheap. In addition, the power furnished to 
consumers can very easily be metered. The loss of energy 
can be kept within moderate limits, and the mains themselves 
are not liable to serious breakdowns, although losses from 
leakage are frequent and may be large. Finally, the system 
is exceptionally safe. On the other hand, the efficiency of 
the system, reckoned to the motor pulleys, is unpleasantly 
low. The mains for a transmission of any considerable length 
are very costly, and the compressed air has no considerable 
use aside from motive power, instead of being applicable, 



60 ELECTRIC TRANSMISSION OF POWER. 

like electric or even hydraulic transmission of power, to divers 
profitable employments quite apart from the furnishing of 
mechanical energy. To obtain a clearer idea of the nature 
of these advantages and disadvantages, let us follow the process 
of pneumatic transmission from the compressor to the motor, 
looking into each stage of the operation with reference to 
its efficiency and economic value. 

The compressor is the starting point of the operation. Fig. 
20 shows in section a typical direct acting steam compressor, 
one of the best of its class. It consists essentially of the air 
cylinder A and a steam cylinder B, arranged in line and having 
a common piston rod. The steam end of the machine is 
simply an ordinary engine fitted with an excellent high speed 
valve gear worked by two eccentrics on the crank shaft of 
the flywheels G, which serve merely to steady the action of 
the mechanism. The air cjdinder A is provided with a simple 
piston driven by an extension of the steam piston rod. 

At each end of the air cylinder are automatic poppet valves 
E E, which serve to admit the air and to retain it during the 
process of compression. F is the discharge pipe for the com- 
pressed air leaving the cylinder. In the compressor shown 
there are two steam and two air cylinders connected with the 
cranks 90° apart, thus giving steady rotation iji spite of the 
character of the work. In some machines the pistons and 
piston rods are hollow and provided with means for maintaining 
water circulation through them, to assist in cooling the air. 
Round the air cylinder is a water jacket shown in the cut just 
outside the cylinder wall. The purpose of this is to keep the 
air, so far as possible, cool during compression, and thus to 
avoid putting upon the machine the work of compressing air 
at a pressure enhanced by the heat that always is produced 
when air is compressed. And just here is the first weak point 
of the compressed air system. How^ever efficient is the 
mechanism of the compressor, all heat given to the air during 
compression represents a loss of energy, since the air loses 
this heat energy before it reaches the point of consump- 
tion. The higher the final pressure which is to be reached, 
the more useless heating of the air and the lower efficiency. 
Hence the water jacket, which, by abstracting part of the heat 



GENERAL CONDITIONS OF POWER TRANSMISSION. 51 

of compression, aids in averting needless work on the air dur- 
ing compression. Even the most thorough jacketing leaves 
much to be desired, generally leaving the air discharged at 
from 200° to 300° F., more often the latter. A cold water 
spray is often used in the compressing cylinder. This is some- 
what more thorough than the jacket, but is still rather in- 
effective. Both serve only to mitigate the evil, since they 
cool the air by absorbing energy from it, and at best cool it 
very imperfectly. A careful series of investigations by Riedler, 
perhaps the best authority on the subject, gives for the efficiency 
of the process of compression from .49 to .72. These figures, 
derived from seven compressors of various sizes and types, 
include only those losses which are due to heat, valve leakage, 
clearance, and the like, taking no account of frictional losses 
in the mechanism. These are ordinarily about the same as 
in a steam engine, say 10 per cent, so that the total efficiency 
of a simple compressor may be taken as .44 to .65, the latter 
only in large machines under very favorable conditions. The 
most considerable recent improvement in compressors is the 
division of the compression into two or more stages, as the ex- 
pansion is divided in compound and triple expansion engines. 
This limits the range of heating that can take place in any 
given cylinder, and greatly facilitates effective cooling of the 
air. Riedler has obtained from two-stage machines of his 
own design a compressor efficiency of nearly .9. Allowing for 
the somewhat greater friction in the mechanism, the total 
efficiency was found to be about .76. In general, then, we maj^ 
take the total efficiency of the single stage compressors usually 
employed in this country as .5 to .6, very rarely higher, while 
the best two-stage compressors may give an efficiency slightl}^ 
in excess of .75. For steady working, .75 would be an excel- 
lent result. 

We may next look into the action of the compressed air in 
the mains. As in the case of water, the frictional resistance 
and consequent loss of pressure vary directly with the square 
of the velocity of the air and inversely with the diameter of 
the pipe. By reducing the one and increasing the other, the 
efficiency of the line may be increased at the cost of a con- 
siderable increase in original outlay. Any attempt to force the 



52 



ELECTRIC TRANSMISSION OF POWER. 



output of the main rapidly increases the losses. At a working 
gauge pressure of 60 pounds per square inch, which is in very 
frequent use, the per cent of pressure lost per 1,000 feet of 
pipe of various diameters is given in the following table — the 
velocity being taken at 30 feet per second: 



Diameter . . . 


V' 


2'' 


3" 


4// 


5" 


6'' 


12'' 


18'' 


24" 


36" 


48" 


Per cent loss . . 


21.8 


10.9 


7.8 


5.45 


4.37 


3.66 


1.0 


0.66 


0.5 


.33 


.25 



The friction in the pipes is proportionally greater in small 
pipes than in large, and this table is taken as correct for the 
medium sizes. No allowance is made for increase in velocity 
through a long main, for leakage, nor for draining traps, 
elbows, curves, and other extra resistances, so that as in prac- 
tice the larger and longer mains suffer the more from these 
various causes, the table will not be found widely in error for 
ordinary cases. Very large straightaway mains will give 
somewhat better results, and the five last columns of the table 
are computed from Riedler's experiments on the Paris air 
mains, llf inches in. diameter and 10 miles long. All losses 
are included. Losses in the air mains can therefore be kept 
within a reasonable amount in most cases. With large pipes 
and low velocities, power can be transmitted with no more loss 
than is customary in the conductors of an electrical system. 
Small distributing pipes, however, entail a serious loss if they 
are of any considerable length. 

The motor is the last element of pneumatic transmission to 
be considered. Generally it is almost identical with an ordi- 
nary steam engine; in fact, steam engines have been often 
utilized for air, and common rock drills may be used indif- 
ferently for steam or air with sometimes slight changes in the 
packing of the pistons and piston rods. Some special air 
motors are in use with slight modifications from the usual 
steam enginie type. In most of these the air is used expan- 
sively and at a fairly good efficiency. Tests by Riedler on the 
Paris system show for the smaller air motors an efficiency of as 
high as 85 per cent so far as the utilization of the available 



GENERAL CONDITIONS OF POWER TRANSMISSION. 53 

energy in the air is concerned, or, taking into account the 
mechanical losses, 70 to 75 per cent. Occasional results as 
low as 50 to 60 per cent were obtained even when the air was 
used expansively, while if used non-expansively the total effi- 
ciency was uniformly below 40 per cent. Tests on an adapted 
steam engine with Corliss valve gear gave a pneumatic effi- 
ciency of .90, with a total efficiency of .81. These figures are 
under more than usually favorable conditions. 

One of the principal difficulties with air motors is freezing, 
due to the sudden expansion of the compressed air, and the 
congelation of any moisture carried with it. It is quite use- 
ful, therefore, to supply to the motor artificially a certain 
amount of heat, sufficient to keep the exhaust at the ordinary 
temperature, especially if the air has been cooled by spray 
during compression. This heating process is very frequently 
extended so as not only to obviate all danger of freezing but to 
add to the output of the air motor by giving to the compressed 
air a very considerable amount of energy. The air is passed 
through a simple reheating furnace and delivered to the motor 
at a temperature of about 300° Fahrenheit. The energy de- 
livered by the motor is composed of that actually transmitted 
through the mains plus that locally furnished by the reheater. 

The amount of fuel used is not great, usually from ^ to | of 
a pound of coal per horse-power-hour, and the increase of 
power obtained is about 25 per cent of that which would 
otherwise be obtained from the motor. This means that the 
heat is very effectively utilized. Reheating is not a method of 
increasing the efficiency of the system, as is sometimes sup- 
posed, but a convenient way of working a hot air engine in 
conjunction with an initial pressure obtained from air mains. 
It increases the operating expense by a very perceptible though 
rather small amount, and gains a good return in power. In so 
far it is desirable, but it no more increases the efficiency of the 
pneumatic transmission than would power from any other 
source added to the power actually transmitted. 

We are now in a position to form a clear idea of the real 
efficiency of transmission of power by compressed air. Taking 
the compressor and motor efficiencies already given, and assum- 
ing 10 per cent loss of energy in the mains, we have for the 



54 ELECTRIC TRANSMISSION OF POWER. 

total efficiency from indicated horse-power at the compressor 
to brake-horse-power at the motor: .75 X .90 x .80 = .54 
for large two-stage compressors and large motors; while with 
ordinary apparatus it would be about .70 X .90 x .75 = .47. 
At half-load these figures would be reduced to about .45 and 
.35 respectively. In operating drills, which are motors in 
which the air is used non-expansively and to which the air is 
carried considerable distances through small pipes, the total 
efficiency is almost always below rather than above .30. The 
efficiency of .54 given above cannot well be realized without 
recourse to artificial heating to enable the air to be used expan- 
sively without trouble from freezing. 

Compressed air has been mainly used for mining operations, 
where its entire safety and its ventilating effect are strong 
points in its favor. More rarely it is employed for general 
power purposes. Of such use the Popp compressed air system 
in Paris is the best and the only considerable example. 

This great work started from a system of regulating clocks 
by compressed air established a quarter-century ago. Nearly 
a decade later the use of the compressed air for motors began, 
and after several extensions of the old plant the present station 
was built. It contains four 2,000 HP compound compressors, 
of which three are regularly used and the fourth held in reserve. 
The steam cylinders are triple expansion, worked with a steam 
pressure of 180 pounds. The air pressure is 7 atmospheres, 
and the new mains are 20 inches in diameter, of wrought iron. 
There are in all more than 30 miles of distributing main, most 
of it of 12 inches and under in diameter. A very large number 
of motors of sizes from a fan motor to more than 100 HP are 
in use. Their total amount runs up to several thousand HP, 
even though the majority of them are less than a single horse- 
power. Except in very small motors, reheaters are used, 
raising the temperature of the air generally to between 200° 
and 300° F. The efficiency of the whole system from Pro- 
fessor Kennedy's investigations is about 50 per cent under 
very favorable conditions. The prices charged for power 
have not been generally known, but are understood to be some- 
what in excess of $100 per horse-power per working year. 
An interesting addition to the apparatus of pneumatic trans- 



GENERAL CONDITIONS OF POWER TRANSMISSION. 55 

mission has recently appeared. It is a modification of the 
ancient 'Hrompe," or water blast, used for centuries to feed 
the forges of Catalonia, very simple in operation and cheap to 
build. In its present improved form it is known as the Taylor 
Hydraulic Air Compressor, and an initial plant of very respect- 
able size has been in highly successful operation for some five 
years past at Magog, P. Q., from which the data here given 
have been obtained. 

The compressing apparatus which is shown in Fig. 21 is in 
principle an inverted siphon having near its upper end a series 
of intake tubes for air, and at the bend a chamber to collect 
the air which, entrained in the form of fine bubbles, is carried 
down with the water column, which flowing up the short arm 
of the siphon escapes into the tail race. In Fig. 21, A is the 
penstock delivering water to the supply tank B. In this 
tank is the mouth of the down tube C, contracted by the 
inverted cone C so as to lower the hydraulic pressure and 
allow ready access of air from the surrounding apertures. 
The air bubbles trapped in the water sweep down C, which 
expands at the lower end, and finally enters the air tank D. 
Here the water column encounters the cone K, which flattens 
into a plate at the base. Thus spread out and escaping into 
the air chamber by the circuitous route shown by the arrows, 
the air bubbles from the water accumulate in the top of the 
air tank, while the water itself rises up the shaft E, and flows 
into the tail race F. The air in D is evidently under a pressure 
due to the height of the water column up to F, and quite 
independent of the fall itself, which consequently may vary 
greatly without affecting the pressure of the stored air, a very 
valuable property in some cases, as in utilizing tidal falls. 
From D the compressed air is led up through a pipe, P, for 
distribution to the motors. To get more pressure, it is only 
necessary to burrow deeper with the air tank, not a diffi- 
cult task where easy digging can be found. The fall and rate 
of flow determine the rate at which the air is compressed, 
and contrary to what might be supposed, the process of com- 
pression is quite efficient. It is quite sensitive to variations 
in the amount of flow, the efficiency changing rapidly with the 
conditions of inlet; and since there certainly is a limit to the 



56 



ELECTRIC TRANSMISSION OF POWER. 





Fig. 21. 



GENERAL CONDITIONS OF POWER TRANSMISSION. 57 

amount of air that can be entrained in a given volume of 
water, the process is Ukely to work most efficiently at moderate 
heads and with large volumes of water. In the Magog com- 
pressor about 4 cubic feet of water are required to entrain 
1 cubic foot of air at atmospheric pressure, and it is open to 
question as to how far this ratio could be improved. This 
ratio, too, would be changed for the worse rapidly in attempt- 
ing high compression, so that the Magog results probably rep- 
resent, save for details, very good working conditions. The 
dimensions of the Magog apparatus are given in the accom- 
panying table, which is followed by the details of one of the 
tests made by a very competent body of engineers. 
The general dimensions of the compressor plant are: 

Supply penstock 60 inches diameter 

Supply tank at top . 8 feet diameter by 10 feet high 

Air inlets (feeding numerous small tubes) 34 2-inch pipes 

Down tube 44 inches diameter 

Down tube at lower end 60 inches diameter 

Length of taper in down tube, changing from 44-inch to 

60-iuch diameter 20 feet 

Air chamber in lower end of shaft 16 feet diameter 

Total depth of shaft below normal level of head water about 150 feet 

Normal head and fall about 22 feet 

Air discharge pipe 7 inches diameter 

Flow of water, cubic feet, minute 4292. 

Head and fall in feet 19.509 

Gross water HP 158.1 

Cubic feet compressed air per minute, reduced to atmos- 
pheric pressure 1148. 

Pressure of compressed air, lbs 53.3 

Pressure of atmosphere, lbs 14.41 

Effective work done in compressing air, HP 111.7 

Efficiency of the compressor, per cent 70.7 

Temperature of external air, Fahr 65.2 

Temperature of water and compressed air, Fahr 66.5 

Moisture in air entering compressor, per cent of saturation 68. 

Moisture in air after compression, per cent of saturation 35. 

The efficiency given is certainly most satisfactory, being 
quite as high as could be attained by a compound compressor 
of the best construction driven by a turbine, and for the head 
in question at a very much lov/er cost. It is probable that 



58 ELECTRIC TRANSMISSION OF POWER. 

the test given does not represent the best that can be done by 
this method, and the indications are that within a certain, 
probably somewhat hmited, range of heads the hydrauHc com- 
pressor will give as compressed air a larger proportion of the 
energy of the water than any other known apparatus. Just 
what its limitations are, remains to be discovered, but several 
plants are now under construction which will throw consider- 
able light upon the subject. 

In certain cases the power of getting compressed air direct 
from hydraulic power by means of a simple and, under favor- 
able conditions, cheap form of apparatus, is very valuable, 
and while it is unlikely to change radically the status of pneu- 
matic transmission, it is an important addition to available 
engineering methods. As in most pneumatic plants, the 
Magog installation is worked in connection with reheaters. 
A similar plant on a somewhat larger scale is in successful 
operation near Norwich, Conn. 

IV. In point of convenience and efficiency, compressed air 
is nearer to electricity for the distribution of power over large 
areas than any other method. The only other system that 
approaches them is the transmission of gaseous fuel for use 
in internal combustion engines. At equal pressures one can 
send through a given pipe twenty times as much energy stored 
in gas as in air. A good air motor requires about 450 cubic 
feet of air at atmospheric pressure per indicated HP hour, 
while a gas engine will give the same power on a little over 20 
cubic feet of gas. But the cases wherein the distribution of 
gas would be desirable in connection with a transmission over 
a long line of pipe are comparatively few. Particularly this 
system has no place in the development of water-powers, the 
most important economic function of electrical transmission. 
Nevertheless it must be admitted that for simple distribution 
of power a well-designed fuel gas system is a formidable com- 
petitor of any other method yet devised, particularly in the 
moderate powers — say from 5 to 25 HP. 

We are now in a position to review the divers sorts of power 
transmission that have been discussed, and to compare them 
with power transmission by electricity. 
Without going deeply into details, which will be taken up in 



GENERAL CONDITIONS OF POWER TRANSMISSION. 59 



due course, we may say that electrical machinery possesses one 
advantage to an unique extent — high efficiency at moderate 
loads. Machinery in which the principal losses are frictional 
is subject to these in amount nearly independent of the load; 
hence the efficiency drops rapidly at low loads. In dynamos, 
motors, and transformers, however, the principal losses de- 
crease rapidly with the load, so that within a wide range of 
load the efficiency is fairly uniform. Fig. 22 gives the efficiency 
curves for a modern dynamo, motor, and transformer. The 



lUO 








^_jrRA 


4SF0RWE 


R 
















/ 


/^ 


^ 


.^'^^'^ 


DVNA 


^0 


== 




90 




> 
o 

z 

Ul 


/ 




^ 


W" 










iZ 
u. 
u 

80 


/ 


















j 
















70 


1 

















J4 



Fig. 22. 



FULL LOAD 



generator curve is from a 200 KW 500-volt direct-current 
machine, the motor curve from a smaller machine of the same 
type, and the transformer curve from a standard type of about 
30 kilowatts capacity. In the generator curve the variation of 
efficiency from half load to full load is less than 2 per cent, 
in the motor only 2\ per cent, and in the transformer just 
\\ per cent. In addition, the efficiency of all three at full 
load is very high. Hence, not only is an electrical power 
transmission of great efficiency if the loss in the line be moder- 
ate, but this efficiency persists for a wide range of load. As 



60 ELECTRIC TRANSMISSION OF POWER. 

in hydraulic and pneumatic transmission, the efficiency of the 
Une depends on its dimensions; so that by increasing the weight 
of copper in the Hne, the loss of energy may be decreased 
indefinitely. And since the loss of energy in the line dimin- 
ishes as the square of the current, the percentage of loss at 
constant voltage diminishes directly with the load. 

Hence, the total efficiency may be constant or even increase 
from half load to full load, even with a quite moderate loss in 
the line. In pneumatic and hydraulic transmission this con- 
dition may occur, but only with large loss in the mains, since 
the efficiencies of the generator and motor parts of such systems 
decrease too rapidly to be compensated by the gain in the 
main, unless its efficiency is low at full load. Hence, for 
ordinary cases of distribution in which the average load is 
considerably less than full load, often only J to ^ of full load, 
electric transmission has a very material advantage over all 
other methods. To appreciate this we need only to run over 
the details of electrical power transmission and compare the 
results with those which we have obtained for the other 
methods described. 

There are to be considered in electrical power transmission, 
as in transmission of every sort, two somewhat distinct prob- 
lems: 

First, the transmission of energy over a considerable dis- 
tance and its utilization in one or a few large units. 

Second, the distribution of power to a large number of small 
units at moderate distances from the centre of distribution. 
This latter case may sometimes also involve the transmission 
of power to a real or fictitious centre of distribution. This 
second problem is the commoner, and, while not so sensational 
as the transmission of power at high voltage over distances of 
many miles, is of no less commercial importance. 

We have all along been considering, in treating of transmis- 
sion of power by ropes and by hydraulic and pneumatic engines, 
the case first mentioned, excepting in so far as some special 
distributions have been referred to. We have already the data 
for figuring the efficiency of an electric power transmission 
with large units. In cases of this kind the distance between 
the generator and motor is likely to be much greater than in 



GENERAL CONDITIONS OF POWER TRANSMISSION. 61 

the case of distribution to small motors from some central 
point, and the loss in the line, the only uncertain figure in the 
transmission, would generally range from 5 to 10 per cent. 
In case of distributing plants intended to furnish from a single 
point small units of power over a moderate distance, it is 
generally found that losses in the line of from 2 to 5 per cent 
do not involve excessive cost of copper. In cases where 
a distribution is coupled with the transmission of power to the 
central point, the loss from the distant generator to the motors 
is in most cases from 10 to 15 per cent. 

Taking up first the transmission of power from one or more 
large generators to one or more large motors, we may take 
safely the commercial efficiency of the generator as that given 
by the curve, Fig. 22, and that of the motors as at least as 
good as that given for a motor in the same figure. The effi- 
ciency of the line for moderate distances may be taken as 
95 per cent. It should be noted that the efficiencies of large 
alternating generators and motors do not differ materially 
from those shown; in fact, are quite certain to be above them. 
We thus have for the efficiency in a transmission of this kind: 
94 X 95 X 93 = 84 per cent. This is largely in excess of 
that which could be obtained at distances of say a couple of 
miles by any other method of transmission. 

Even more extraordinary is the efficiency at half load in 
this case, which is 92 x 97.5 x 91 = 81.6 per cent. It 
should be borne in mind that these efficiencies are taken from 
experiments with ordinary machines, and the efficiencies are 
those which can be bettered in practice. These results show 
the great advantage to be derived from electrical transmission 
when, as in most practical cases, full load is seldom reached. 
It is most important for economical operation to employ a 
system which will give high efficiency at low loads, and it 
would be worth while so to do even if the efficiency at full 
load were not particularly good. With electrical machinery, 
however, there is no such disadvantage. Even at one-fourth 
load the efficiency of the electrical system still remains good. 
It is nearly 73 per cent on the assumed data. The efficiencies 
thus given are from the shaft of the generator to the pulley 
of the motor inclusive. 



62 



ELECTRIC TRANSMISSION OF POWER. 



In the case of distributed motors supplied from a central 
point not very distant from any of them, the efficiencies of 
generator and line remain about as before, but the motor 
efficiencies for the sizes most often employed are below that 
just given. The average motor efficiency is largely dependent 
on the skill with which the units are distributed. It has often 
been proposed to drive separate machines by individual motors, 
while in other cases comparatively long lines of shafting are 
employed, grouping many machines into a dynamical unit 
operated by a motor. To secure economy it is desirable on 
the one hand to use fairly large motors well loaded, while on 
the other hand the losses in shafting and belting must be kept 
down. 

The larger the motors, the better their efficiency at all 
loads and the less the average cost per HP, but with small 
motors the cost and inefficiency of shafts and belts may be 
in large measure avoided. The most economical arrange- 
ment depends entirely upon the nature of the load. Much 
may be said in favor of individual motors for each machine, 
but so far as total economy is concerned, this practice is best 
limited to a few cases — machines demanding several HP (say 
5 or more) to operate them, machines so situated as to neces- 
sitate much loss in transmitting power to them, and certain 
classes of portable machines. In applying electric power 
to workshops already in operation, the group system will 
usually give the best results, individual motors being used 
only for such machines as might otherwise cause serious loss 
of power. The following table gives the average full load 
efficiencies that may safely be expected from motors of various 
sizes, irrespective of the particular type employed. 



HP of motor . . 


1 


3 


5 


n 


10 


15 


20 


25 


40 


50 


75 


Per cent efficiency 


72 


78 


81 


83 


85 


86 


87 


88 


90 


90 


92 



These are commercial efficiencies reckoned from the electri- 
cal input to the mechanical output at the pulleys. Below 5 HP 
the efficiencies fall off rapidly. At partial loads the efficiencies 



GENERAL CONDITIONS OF POWER TRANSMISSION. 63 

are somewhat uncertain, inasmuch as some motors are designed 
so as to give their maximum efficiency at some point below 
full load, while others work with greater and greater efficiency 
as the load increases until heating or sparking limits the out- 
put. The former sort are most desirable for ordinary work- 
shop use, while the latter are well suited to intermittent work 
at very heavy loads, as in hoisting. The difference in the two 
types of machine is very material. It is easily possible to 
procure motors that will not vary more than 5 per cent in 
efficiency from full load to half load, and this even in machines 
as small as 2 or 3 HP. We may now calculate the efficiency 
of an electric distribution with motors of moderate size — such 
a case as might come from the electrical equipment of large 
factories. The generator efficiency may be taken as before 
at .94 and that of the line at .95, while the motors must be 
taken close account of in order to estimate their collective 
efficiency. Assuming the sizes of motors in close accordance 
with those in several existing installations of similar character, 
we may sum them up about as follows: 

5 3 HP 

5 5 HP 

10 10 HP 

10 20 HP 

5 25 HP 

2 50 HP 

In all 37 motors, aggregating 565 HP. The mean full load 
efficiency of this group is very nearly .87. The efficiency of 
the system is then 

.94 X .95 X .87 = 77.6. 

This result requires full load throughout the plant, a some- 
what unusual condition with any kind of distribution. From 
the data already given, the half load efficiency should be about 

.92 X .975 X .82 = .735. 

Between the limits just computed should lie the commercial 
efficiency of any well-designed motor distribution reckoned 
from the dynamo pulley. In the case of steam-driven plants 
it is often desirable to consider the indicated HP of the engine 
as the starting point, and the question immediately arises as 



64 



ELECTRIC TRANSMISSION OF POWER. 



to the commercial efficiency of the combination of dynamo 
and engine. In cases where high efficiency is the desideratmn 
direct couphng is usually employed, saving thereby the loss 
of power, perhaps 5 per cent, produced by belting. The 
losses in such direct-coupled units vary considerably with the 
size and type of both machines. Fig. 23 shows the efficiency 
of two such combinations at various loads. Curve A is from 
an actual test of the combination; curve B from tests of an 
engine and dynamo separately. Each unit was of several 



IGO 



90 





























^ 




:=^ 






/ 


^ 












^ 


/ 


y 










A 


/ 


B 












/ 

















;70 



50 



40^ 



% FULL LOAD 



Fig. 23. 



hundred HP. The high result from curve A is mainly due 
to very low friction. 

These curves give handy data for computing the total effi- 
ciency of a motor plant from the motor pulleys to the indicated 
horse-power of the driving engine. Taking the combined 
engine and dynamo efficiency from A, and assuming the same 
figures as before on motors and line, we have at full load, 

.88 X .95 X .87 = .727. 
And from the same data at half load, 

,78 X .975 X ,82 = .651. 



GENERAL CONDITIONS OF POWER TRANSMISSION. 65 

For certain computations, as in case of figuring out a com- 
plete installation, the above efficiencies are convenient. They 
show that in very many instances the distribution of power by 
electric motors is very much more economical of energy than 
any other method employed. In ordinary manufacturing 
operations power is generally transmitted to the working 
machines through the medium of lines of shafting of greater 
or less length. These are very rarely belted directly to the 
machines, but transfer power to them through one or more 
countershafts. Often the direction of shafts is changed by 
gearing or quarter turn belts, and even when the power is 
distributed through only a single large building there will be 
found more often than not, intervening between the driving 
engine and the driven machine, three belts and two lines of 
shafting of considerable length, and not infrequently still other 
belts and shafts. It very often happens, too, that to keep in 
operation one small machine in a distant part of the shop, it is 
necessary to drive a long shaft the friction of which consumes 
half a dozen times as much power as is actually needed at the 
machine. The constant care required to keep long lines of 
shafting in operative condition is an irritating and costly con- 
comitant. The necessary result is a considerable loss of power, 
which, being nearly constant in amount, is very severe at 
partial loads. 

Allowing 5 per cent loss of energy for each transference 
of power by belting, a figure in accordance with facts, and 10 
per cent loss for each long line shaft driven, it is sufficiently 
evident that from 20 to 25 per cent of the brake-horse-power 
delivered by the engine must be consumed even under very 
favorable circumstances by the belting and shafting at full 
load. This means an efficiency at half -load of from 50 to 60 
per cent only, and at lesser loads a very low efficiency indeed. 

The large number of careful experiments carried out on 
shafting in different kinds of workshops, and under various 
conditions, sho ws that only under very exceptional circum- 
stances is the loss of power by shafting between the engine 
and the driven machines as low as 25 per cent. Far more 
often it is from 30 to 50 per cent, and sometimes as high as 
75 or 80 per cent. The figures, which have been well estab- 



66 ELECTRIC TRANSMISSION OF POWER. 

lished, regarding the efficiency of the transmission of power 
by motors, show that at full load it is comparatively easy 
to exceed 75 per cent efficiency; thus more than equalling the 
very best results that can be obtained with shafting. At half- 
load and below, the advantage of the electric transmission be- 
comes enormous, even supposing shafting to be at its very 
best. 

Compared with ordinary transmission by shafting, the motor 
system is incomparably superior at all loads, so that it may 
easily happen that a given amount of work can be accomplished 
through the medium of a motor plant with one-half the steam 
power required for the delivery of the same power through 
shafts and belts. Such results as this have actually been 
obtained in practice. It is, therefore, safe to conclude that 
the distribution of power by motors is, under any ordinary 
commercial conditions, at least as efficient as the very best 
distribution of power by shafting at full load, and much more 
efficient at low loads. Under working conditions in almost all 
sorts of manufacturing establishments, light loads are the rule 
and full loads the rare exception; consequently the results of 
displacing shafting by motor service have, as a rule, been 
exceedingly satisfactory in point of efficiency, and the lessened 
operating expense more than offsets the extra cost of installa- 
tion. 

In one early three-phase plant, that of Escher, Wyss & Co. 
at Winterthur, Switzerland, 300 HP in 32 motors worked from 
a 12-mile transmission line displaced far greater capacity in 
steam engines, and similar results on a smaller scale are not 
uncommon. 

To add force to this comparison between the efficiency of 
shafting and of motors, the following results from electrical 
distribution plants already installed may be pertinent. A 
typical example of the sort is from a plant installed some years 
since in a fire-arms factory at Herstal, Belgium. There were 
there installed 17 motors of an aggregate capacity of 305 HP, 
driven by a 300-KW generator direct-coupled to a 500-HP 
compound-condensing engine. The efficiency guaranteed from 
the shaft of dynamo to the pulleys of the motors is 77 per cent. 
Since its first installation, the plant has been increased by the 



GENERAL CONDITIONS OF POWER TRANSMISSION. 67 

addition of a second direct-coupled dynamo and the total horse- 
power of motors is 428. A second notable installation of 
motors in the same vicinity is at the metallurgical works of 
La Societe de la Vielle-Montagne, consisting of a 375-KW 500- 
volt dynamo direct driven at a speed of 80 revolutions per 
minute by a 600-HP compound-condensing engine. The plant 
consists of 37 motors with an aggregate HP of 329. The full 
load efficiency of the plant from dynamo shaft to motor pulley 
is 76 per cent. The loss in the lines, both in this case and 
in the preceding, is very small, only 2 per cent. They are 
both typical cases of transmission to motors driving groups 
of machines, and in spite of rather low dynamo efficiencies 
at full load, these being in each case 90 per cent, the results 
obtained are in close accordance with those already stated as 
appropriate to similar cases. As an example of work under 
more favorable conditions, the early three-phase power plant 
at Columbia, S. C, may be instanced. 

The problem here undertaken was to drive a very large cot- 
ton mill, utilizing for the purpose a water-power about 800 feet 
distant. Two 500-KW dynamos direct-coupled at a speed of 
108 turns per minute deliver current at 550 volts to an under- 
ground line connecting the power station with the mills. 
The motors are suspended from the ceiling, and each drives 
several short countershafts. The motors are wound for the 
generator voltage without transformers, and are of a uniform 
size, 65 HP each. The commercial efficiency of this plant, 
taken as a whole from the shaft of the dynamo to the pulleys 
of the motors, is not less than 82 per cent at full load. This 
good result is due to the use of large motors, and to the small 
line loss of 2 per cent as in the preceding foreign examples. 
These results are thoroughly typical, and can regularly be 
repeated in practice. In general a net efficiency of 80 to 85 
per cent can be counted upon in plants of the approximate 
size of those here mentioned, assuming the apparatus now 
commercially standard. Even smaller plants can be counted 
on to give nearly or quite as good results, since the differ- 
ence in efficiency, supposing motors of the same size to be 
used, between a dynamo of 100 KW and one of 400 or 500 KW 
is hardly more than 1 per cent at full load, supposing machines 



68 ELECTRIC TRANSMISSION OF POWER. 

of the same general design to be employed, nor is there any 
substantial difference in efficiency between plants employing 
direct current and those using polyphase apparatus, as may 
be judged from the figures just given. 

We are now in position intelligently to compare the trans- 
mission and distribution of power by electric means with the 
other methods which have sometimes been employed. 

All comparisons between methods of transmitting power 
have to be based in a measure on their relative efficiency. 
Now, in every such method there are three essential factors: 
1st, the generating mechanism, which receives power direct 
from the prime mover and in conjunction with which it is 
considered; 2d, the transmitting mechanism, which may be an 
electric line, a pipe line, ropes, or belts; and 3d, the motor 
part of the transmission, which receives power from the trans- 
mitting mechanism and delivers it for use. For a given 
capacity of the generating and receiving mechanisms, the 
efficiency of each at all loads is determined within fairly close 
limits. The transmitting mechanism, however, is not so 
closely determined, save in the case of the rope drive. 

Electric, pneumatic and hydraulic transmission lines are all 
subject to the general principle that the loss in transmission 
can be made indefinitely small by an indefinitely large expen- 
diture of capital, enormous cross-section in the one case, or 
huge pipe lines in the others. The efficiency of these methods 
is, therefore, a fluctuating quantity depending on that loss in 
the transmitting mechanism which may be desirable from an 
engineering or economical standpoint. In making compar- 
isons between these methods, there is a wide opportunity for 
error unless some common basis of comparison is predeter- 
mined. In the next case any such comparison must differ 
widely in its results according to the character of the power 
distribution which is to be attempted. We have already seen 
that with the rope drive, distribution is very difficult, while 
with electric and pneumatic systems it is comparatively 
easy. 

A general valuation of the commercial possibilities of these 
divers matters is, therefore, hard to make except in a general 
way. We can, however, by assuming a given transmission of 



OENERAL CONDITIONS OF POWER TRANSMISSION. 69 

given magnitude and character, and further assuming such 
loss in the transmitting mechanism as might reasonably be 
expected in practice, arrive at a reasonably accurate conclu- 
sion for the case considered. As a very simple example of 
power transmission, let us take the delivery of power over a 
distance of two miles, the delivery being in one unit or, at 
most, two units. We will assume the same indicated HP fur- 
nished at the generating end of the line in each case, of which 
as much as possible is to be delivered at the receiving station, 
the losses in transmission being taken as 10 per cent of the 
power delivered to the line; this is to cover all losses of 
energy by resistance and leakage on the electrical line or 
loss of pressure and resulting expenditure of energy, leakage, 
friction, and all other sources of loss in the other cases. 

As the same indicated power is generated in each case, we 
will suppose a modern plant with compound condensing en- 
gines costing complete with buildings $50 per HP. We will 
further assume that each indicated horse-power per working 
year of 3,000 hours will cost $18; this covering all expenses 
except those chargeable to interest and depreciation. For 
this simple case we have the following costs of initial plant 
and of operation per mechanical horse-power delivered from 
the motor, full load only being considered. The four meth- 
ods considered are rope driving, pneumatic, pneumatic with 
reheating apparatus at the motors, and electrical. The prices 
are from close estimates of the cost in each case. The dyna- 
mos are supposed to be direct-coupled. The compressors to 
be direct-acting, two-stage compressors. The steam cylinders 
Corliss compound-condensing type. The air-pressure assumed 
is 60 lbs. above atmospheric pressure. The electric voltage 
3,000. The rope speed about one mile per minute. Interest 
and depreciation are taken at 10 per cent of the total cost of 
the plant, save in the case of the rope drive, where an addi- 
tional charge for renewal of cable is made on the supposition 
that the cable will last somewhere from 18 months to 2 years, 
which is fully as favorable a result as can fairly be expected. 
The following are the comparative estimates: 



70 ELECTRIC TRANSMISSION OF POWER. 

Rope, Efficiency 67 per cent. 

COST. 

Steam plant $60,000 

Pulley stations 25,000 

Cables, steel 17,000 

Total cost $92,000 

operating expense. 

1,000 I.HP at 118 $18,000 

Interest and depreciation on plant, at 10 per cent . . . 7,500 

Depreciation of cable 8,000 

$33,500 
Net HP produced, 672. 
Cost per HP-year, $49. 

Pneumatic, Efficiency 54 per cent. 

COST. 

Steam plant, excluding engines $35,000 

Compressors ........... 17,000 

Air mains laid, 12 inches 18,000 

Air motors 12,000 

Total cost $82,000 

OPERATING expense. 

1,000 I.HP at $18 $18,000 

Interest and depreciation, at 10 per cent 8,200 

$26,000 
Net HP delivered, 540. 
Cost per HP-year, $48. 

Air Reheated, Apparent Efficiency 65 per cent. 

COST. 

Steam plant, excluding engines $35,000 

Compressors 17,000 

Air mains laid 18,000 

Air motors and rebeaters with chimney, etc. .... 14,000 

Total cost $84,000 

operating expense. 

1,000 I.HP at $18 $18,000 

Interest and depreciation, at 10 per cent 8,400 

Coal and labor for reheating 1,500 

$27,900 
Net HP delivered, 650. 
Cost per HP-year, $43. 



GENERAL CONDITIONS OF POWER TRANSMISSION. 71 



Electric, Efficiency 73 per cent. 

COST. 

Steam plant " $60,000 

Dynamos 18,000 

Line 3,000 

Motors 13,000 

Total cost $84,000 

OPERATING EXPENSE. 

1,000 I.HP at $18 $18,000 

Interest and depreciation, at 10 per cent ..... 8,400 

Electrician , . . . . 1,500 

$27,900 
Net HP, 730. 
Cost per HP-year, $38. 

It appears at once that the rope drive is beyond the range of 
its efficient use. Its first cost is greater than that of either of 
the other methods, and the expense is carried to a very high 
figure by the item of depreciation on the cables, which cannot 
be avoided; hence in spite of a high efficiency, the cost per 
HP year dehvered rises to $49. We may next consider the 
schedule of cost for the pneumatic system. In this case the 
most formidable item is the cost of the air-mains, which should 
be at least 12 inches in diameter. Nevertheless, the total initial 
cost is the lowest of the four. The operating expense is also 
the lowest, but. the very low efficiency of the pneumatic system 
without reheating raises the cost per HP delivered to a very 
considerable amount — almost as much as in the case of the 
rope drive. Reheating would almost always be used in con- 
nection with a plant of this size, and with reheating the result 
is much more favorable. The initial expenditure is somewhat 
increased by the addition of the reheaters, piping and chimney. 
The operating expense is also slightly increased by the coal 
necessary for reheating, taken at ^ of a pound per HP per hour, 
and the small amount of additional labor involved in caring 
for the reheaters, disposing of the ashes, and looking after 
the reheating plant generally. The apparent efficiency in 
this case is very excellent, 65 per cent being reasonably attain- 
able, and the cost per HP year falls to $43, shov/ing conclu- 



72 ELECTRIC TRANSMISSION OF POWM. 

sively enough the advantage of reheating; at least, where 
the units are so large that the presence of a reheater is not 
a practical nuisance. 

Finally, we come to the electric power transmission. In 
this case the most striking feature is the low cost of the line, 
supposed here to be overhead. It may be noted, however, 
that an underground line, consisting of cable laid in conduit, 
still leaves the cost per HP year lower than that of any of 
the other methods. Operating expense is fairly increased 
by the addition of an electrician to the cost of the indicated 
horse-power, interest, and depreciation. The total first cost 
is practically the same as that of air with reheater, as is also 
the operating expense. The added efficiency, however, brings 
the cost per HP year to $38; decidedly the lowest of the four 
cases considered. It may be thought that difference of loss 
in transmission might possibly alter the relation of the electric 
plant to the air-plant with reheaters, but an added efficiency 
of line would in either case be accompanied by added expen- 
diture of not very different amounts in the two cases, and the 
efficiency of the electric plant would always be enough higher 
than that of the air-plant to give it the advantage in net cost 
per HP, however the two plants might be arranged. We thus 
find that at a distance of two miles the electric transmission 
has a material advantage, air with reheaters, air without 
reheaters, and rope drive following it in the order named. 
The pneumatic method would at the distance of one mile, 
as may readily be computed, take about the same relative 
position as before, since the efficiency maintains approxi- 
mately the same relation to the others. 

The pneumatic plant gains in first cost at this lesser dis- 
tance, not enough, however, to alter the final result. At 
half a mile distance, the rope drive will be found to be the 
cheapest in first cost, and also, through its enormous efficiency, 
to be a little the cheapest per HP delivered, in spite of the 
large depreciation in the cables, while the electric and pneu- 
matic systems would be very close together, the electric, 
however, still retaining a slight advantage due to its greater 
efficiency. Neither can, in point of absolute cost of power 
delivered, compete with the rope drive at this distance for 



GENERAL CONDITIONS OF POWER TRANSMISSION. 73 

this simple transmission at full load, although both would 
surpass it were there any considerable distribution of the 
power. Figures that have heretofore been given on the 
relative cost and efficiency of such transmissions have as a 
rule been in error in two very essential particulars: first, the 
efficiencies of the electrical system have been greatly under- 
estimated owing to the poor machines with which the first 
experiments were made; second, the commercial advantage of 
reheating in the pneumatic transmission has not generally been 
given its proper weight. It is, as has been already stated, not 
a method of increasing the efficiency, but of increasing the 
power delivered by addition of energ}^ at the receiving end of 
the line under very favorable conditions. The figures just 
given are believed to be as nearly exact as roughly assumed 
conditions permit. 

One modification in the electric transmission should here 
be noted. The recent introduction of the steam turbine has 
rendered it possible to lower the cost of the generating plant 
very materially, while retaining a cost of power at the prime 
mover not in excess of that here given. The generating unit 
also comes in for some reduction in the combined frictional 
loss, so that the final cost per HP year would on the basis here 
taken probably fall to $35 or $36, giving the electrical system 
a still greater advantage. This statement does not mean 
broadly that a turbo-generator can regularly deliver power 
six or eight per cent cheaper than an ordinary generating 
set, but merely that it would probably do so under the cir- 
cumstances here assumed. 

All these estimates are subject to change of prices from 
year to year, but no changes are likely to be sufficient to alter 
the relative position of the methods compared as regards 
cost. It is not a difficult matter to construct a set of estimates 
arranged to favor any given method. In the long run, what- 
ever minor variations may appear in the items here given, 
the totals will be found to scale up or down in about the same 
ratios. 

At less than full load and hence under variable loads, the elec- 
tric system enjoys the unique advantage of having the losses 
of energy in every part of the system decrease as the load 



74 ELECTRIC TRANSMISSION OF POWER. 

decreases, while in rope driving all the losses are practically 
constant, and in the hydraulic and pneumatic systems all 
are nearly constant save that in the pipe-line. 

Hence, under low and varying loads, electric transmission has 
a great additional advantage. Since in distributions of power 
employing a considerable number of motors light load on the 
motors is the invariable rule, as soon as we depart from the 
very simple case discussed the electrical system gains in rela- 
tive economy at every departure. These more general cases 
have already been described, and gathering the results we 
may construct the following table, showing the efficiency of 
each svstem under full and half-loads: 



System, Full Load. Half Load 



Wire rope 67 46 

Hydraulic high pressure 53 45 

Hydraulic low pressure 50 50 

Pneumatic 50 40 

Pneumatic reheated (virtual efficiency) . 65 50 

Electric 73 65 



The efficiencies in the electric system as here given are 
lower than would be reached practically in large plants. The 
present practice of using generators and motors wound fo^ 
pressures up to 10,000 or 12,000 volts makes a most material 
difference in the matter of efficiency. For a well-designed 
transmission of a few miles in units of say 500 KW and up- 
wards, one may fairly expect to get at full load as much as 94 
per cent from generator and motor, and perhaps 98 per cent 
from the line, giving a total efficiency of transmission of 

.94 X .94 X .98 = .866 

at full load and of nearly .85 at half-load or say 79 and 72 per 
cent respectively when reckoning efficiency from the indicated 
HP of the engine as in the foregoing comparisons. This means 
far higher efficiency than can be obtained by any other method 
at any but the shortest distances. 

All the figures must be taken as approximate. They are 
under conditions fairly comparable except in case of the low- 




GENERAL CONDITIONS OF POWER TRANSMISSION. 75 

pressure hydraulic system, in which the large proportion of 
loss due to pipe-friction operates to hold up the half-load effi- 
ciency to an abnormal degree. With the ordinary proportion 
of small motors this half-load efficiency would be nearer 40 
than 50 per cent. The electric system is easily the most 
efficient at any and all loads. Of the others, wire-rope trans- 
mission, if the distributed units are fairly large, holds the 
second place for short distances, and the pneumatic system 
with energy, added at the motors by reheating, at moderate 
and long distances. Without reheating it occupies the last 
place in order of efficiency, although even so, it is, next to 
electricity, the most convenient method of distributing power. 

In fact, electricity and compressed air are the only two 
systems available for the general distribution of energy. The 
latter is, save for a single system in Paris, used only on a small 
scale, and in this country hardly at all save in mining. Of 
course, the very largest power stations are those belonging to 
electric railway systems in the largest American cities. Several 
of these exceed 50,000 HP in generator capacity and frequently 
in actual output, notably the systems in Boston, Brooklyn, 
and Philadelphia. Recent advances in electrical engineering, 
particularly the effective utilization of alternating currents, 
have greatly cheapened the distribution of electrical energy, 
and other systems are now^ seldom installed for ordinary pur- 
poses. A few pneumatic and hydraulic plants will continue 
to be used, owing to the large capital already invested in them, 
but new work is, and in the nature of things must be, almost 
exclusively electrical. As the transmission of power from 
great distances becomes more common and the radii of dis- 
tribution themselves increase, the electrical methods gain more 
and more in relative value, and all others become more ineffi- 
cient and impracticable. 

We have now discussed in some detail the sources of natural 
energy which are available for human use, and the most promi- 
nent of the systems employed for their utilization. We have 
found that for practical purposes steam-power and water- 
power must at present be used to the virtual exclusion of all 
others, the former perhaps less than the latter save for dis- 
tribution of power over short distances. 



76 ELECTRIC TRANSMISSION OF POWER. 

Of the methods of distribution we have found all save com- 
pressed air and electricit}^ very limited in their application, 
the hydraulic systems to special classes of work under favorable 
topographical conditions, and rope transmission to extremely 
short distances and small numbers of power units delivered. 
Both are noticably inefficient. The pneumatic system is very 
general in its applicability, but of very low intrinsic efficiency. 
Wher^ used in connection with reheating apparatus it requires 
additional care, and the motors, like steam-engines, are heavy 
and inconvenient. The electric system on the other hand em- 
ploys motors which are compact and far more efficient than 
any other type of machine for delivering mechanical power, 
run practically without attention, and can be placed in any sit- 
uation or position that is convenient. Furthermore, in average 
working efficiency the electric system is 10 to 15 per cent 
higher than any other yet devised, so that it is more economical 
in use at nearly all distances and under nearly all conditions. 
Finally, it unites with power distribution the ability to furnish 
light and heat, thus gaining an immense commercial advan- 
tage. This advantage is shared only by gas transmission, 
which up to the present time remains of doubtful value on 
account of the cost of the motors, their imperfect regulation, 
and their inefficiency at moderate loads. Gas transmission, 
however, is likely to grow in importance, owing to the great 
improvements in small gas-engines stimulated by the rapid 
development of the automobile. Having now overlooked 
the advantages of electrical power, it is proper to pass to the 
details of the methods employed for its utilization, and thence 
to the general problem of its economical generation, trans- 
mission and distribution. 



CHAPTER III. 

POWER TRANSMISSION BY CONTINUOUS CURRENTS. 

Up to the present time the larger part of electrical power 
transmission has been done by continuous currents. All the 
earlier plants were of this type, and even now, when trans- 
mission by alternating currents, polyphase and other, is gen- 
erally used, the older type of apparatus is still being installed 
on an extensive scale, and on account of the large number of 
plants now in operation, even if for no other reason, will prob- 
ably remain in use for a long time to come. New power 
transmission plants, both here and abroad, are, save for rare 
exceptions, for alternating currents, and in many cases this 
practice is almost absolutely necessary, but there still remain 
many cases wherein the conditions are as well met in the old- 
fashioned way. 

Chief among these may be mentioned electric railway work, 
which in America alone certainly requires more than a full 
million horse-power in generators and motors. Certain diffi- 
cult work at variable speed and load, and many simple trans- 
missions over short distances, are at present best handled by 
continuous current machinery. As alternating practice ad- 
vances, many, perhaps all, of these special cases will be elimi- 
nated, but we are dealing with the art of power transmission as 
it exists to-day, and hence continuous current working deserves 
consideration. 

The broad principle of the continuous current generator has 
already been explained, but its modifications in actual work 
are important and worthy of special investigation. In a gen- 
eral way, continuous currents are almost always obtained by 
commuting the current obtained from a machine which would 
naturally deliver alternating currents. This process is, how- 
ever, by no means as simple as Fig. 9 would suggest. With a 
two-part commutator the resulting current, although unidirec- 
tional, would necessarily be very irregular, owing to the fact 

77 



78 



ELECTRIC TRANSMISSION OF POWER. 



that the total current drops to zero at the moment of com- 
mutation. Such a current is ill fitted for many purposes, and 
the commutator would be rapidly destroyed by sparking if the 
machine were of any practical size. 

To avoid these difficulties, the number of coils on the arma- 
ture is increased, and they are so interconnected that, while 
each coil has its connection to the outside current reA^ersed as 
before, when its electromotive force is zero, the other coils in 
which the E. M. F. still remains in the right direction continue 
in circuit unchanged. In this way the E. M. F. at the brush 
is the sum of the E. M. F.'s of a number of coils, each of which 
is reversed at the proper moment. The number of commutator 
segments is increased proportionally to the number of coils, 




Fig. 24. 



and the commutator thus becomes a comparatively complicated 
structure. The result, however, is that the total E. M. F. of 
the armature cannot vary by more than the variation due to a 
single coil. The nature of this modification is shown in Fig. 
24, which shows a four-part commutator connected to a four- 
coil drum armature. 

An eight-part winding of modern type is shown in Fig. 25. 
Tracing out the currents in this will give a clear idea both of 
a typical winding and of the process of commutation. 

In commercial machines the number of individual coils and 
of commutator segments often exceeds 100, but the principle 
of the winding is the same. Nearly all the early dynamos had 
several turns of wire per coil, as in Fig. 24, but at present, in 
most large machines, one turn constitutes a complete coil. 



TRANSMISSION BY CONTINUOUS CURRENTS. 



79 



This subdivision is to avoid sparking at the commutator, 
which becomes destructive if the current be large and the 
E. M. F. per commutator segment more than a few volts. 

If each coil generates a considerable voltage, there is even 
under the best conditions of commutation a strong tendency 
for sparks to follow the brush across the insulation between 
segments, or even to jump across this insulation elsewhere. 
As this goes from bad to worse, and rapidly ruins the commu- 
tator, every precaution has to be taken against such a con- 
tingency. The E. M. F. generated by each coil is kept low 
by subdividing the winding, and in large machines it is the 




Fig. 25. 



rule that the E. M. F. of a single loop is quite all that can 
safely be allotted to a single commutator segment. 

Present good practice indicates that, in generators for light- 
ing, up to 100 or 150 volts the voltage between brushes should 
be subdivided so that it shall not exceed 3 or 4 volts for 
each segment between the brushes. For 500 or 600 volt 
machines it should not ordinarily exceed 8 or 10 volts, while 
for dynamos of moderate output and even higher voltage it 
may rise to 20 volts or more. 

The reason for these different figures is that the destructive^ 



80 



ELECTRIC TKAXSMISSIOX OF PDWER. 



noss of the spark depends on the amount of current which is 
liable to be involved. On a low voltagt^ commutator intended 
for heavy currents, even very moderate sparking may gnaw 
the segments seriously, wliile the spark of an arc machine, in 
spite of its venomous appearance, n\ay do very little harm, {\s 
the maximum current in the whole bar will not exceed 8 or 10 
ampoix^s. Consequently the voltage per bar in such cases is 
^sometimes 50 or moi-e, while in very large incandescent ma- 
cliines, and in those designed for electrolytic purposes, the 
K. M. V. per bar is often less than 2 volts or even below 1 volt. 
^Viudinirs like tht^se of Fiirs. 24 and 25 ai\} of the so-called 




Fu;. iV. 



dratn type, in which each convoliuion oxioiuis around the whole 
body of the armatinw either dian\etrically or nearly so. 
Another sort of arinniuro winding frequently used, although 
less now than formerly, is the Gramme, so called fron\ its 
inventor. Here the iivu body of the anuature is. instead 
of being cylindrical, in the form of a massive rinc oi rect- 
angular cross-section. The wii\dings are looped through and 
around this ring, fitting it tirmly and closely, l^^g. 2t>. which 
shows in viiairrani a winding in ten sections, furnishes a good 
example of the dramme construction. Theix* nu\y be one or 
several turns per coil, as in drum windings. These two gen- 
eral ty}H^s oi windings arc used with various modifications in 



TRANSMISSION BY CQXTIXUOUS CURRENTS. 81 

nearly all continuous current dynamos. Each has its good 
and bad features. The Gramme Tsinding makes it ver\" easy 
to keep do^^Tl the voltage per segment^ inasmuch as for each 
external armature wire there is a commutator bar. while in 
the drum form there is but one bar for two wires. It is also 
mechanically solid even when wound \^'ith small wire, and no 
two adjacent wires can have a considerable voltage between 
them, thus making it easy to build an armature for high 
E. M. F. On the other hand, the drum \dnding gives a very 
compact armature of easy construction, and the magnetism 
induced in it is less likely to disturb that of the field. 

In the small machines once usual, the Gramme tj-pe was pre- 
ferred for high voltages on account of the ease \sith which it 
could be repaired, while the drum was liked for its simplicity 
of mechanical construction as a whole and excellent efficiency 
as an inductor. In modem practice the differences between 
these types have become much less marked. With large units, 
particularly of the multipolar form now usual, the drum grind- 
ing is as easily insulated as the Gramme, for \\ith the \\inding 
now used in such cases there need be no considerable voltage 
between adjacent wires, and repairs are of very infrequent 
occurrence. In fact, the drum \^-inding can be made quite as 
accessible as the other, and is on the whole cheaper and 
simpler. Almost the sole advantage of the Gramme (or ring) 
winding is that of low voltage per commutator bar. Mechan- 
ically, too, there is less difference than formerly, for the coils 
are in both types generally bedded in slots in the iron of the 
armature core. 

It must be noted that the armature of the modem d}Tiamo, 
unless of small size or unusually high voltage, is seldom wound 
with wire in the ordinary sense of the word. Instead, the 
conductors are bars of copper, usually of sections rectangular 
rather than round, and generally lacking any permanently 
attached insulation. Whatever the winding, the conductors 
on the armature face are inclosed in close fitting tubes of 
mica and specially treated paper or the like, and then put 
in place on the armature core or in more or less completely 
closed channels cut in it. If on the core surface, the bars are 
often not insulated on the exterior surface at all. If the arma- 



82 



ELECTRIC TRANSMISSION OF POWER. 



ture core be slotted, the insulating material is preferably put 
in position first and the bar put in afterward. As to the rest 
of the winding, it is completed by connectors of copper strip or 
rod soldered to the face conductors and insulated in a substan- 
tial manner. Thus each convolution, whether of ring or drum 
winding, is composed of from two to four pieces. 

A typical modern ring winding is shown in Fig. 27. It well 
exemplifies the construction above mentioned, and in this case 
the uninsulated faces of the exterior conductors form the com- 
mutator of the machine. Such a construction of course ex- 




Fig. 27. 



eludes iron clad armatures, and is best fitted for a machine 
having a field magnet inside the ring armature. A similar 
arrangement which avoids the above limitations, uses the side 
connectors of the ring as commutator segments. The general 
principle, however, is the same, whether the commutator 
forms part of the winding proper or is a separate structure. 

An iron clad drum winding of typical character is shown in 
Fig. 28. Here the exterior bars are fitted into thoroughly 
insulated slots in the core, and wedged firmly into place by 
insulating wedges. Sometimes the bars themselves are shaped 



TRANSMISSION BY CONTINUOUS CURRENTS. 83 

so as to act as wedges. In either case the bars are held almost 
as solidly as if they formed an integral part of the core. The 
commutator in these windings must be a separate affair. Fig. 
28 shows well the nature of the winding, with its slotted core, 
ventilating spaces, and massive bars — in this example 4 per 
slot. The end connectors lie in a pair of reverse spirals, one 
outside the other, and separated by firm insulation. The 
relation of these connectors to the rest of the winding is illus- 
trated in Fig. 25. 

Between the modern drum and ring armatures it is needless 
to discriminate. Both have been successfully used in dynamos 
of the largest size, but the iron-clad drum is in the more gen- 
eral use, while the use of ring armatures is steadily declining. 




Fig. 28. 

It is very unusual to find a standard generator of recent build 
of 100 KW or more output with a regular wire wound arma- 
ture, and the most of them have some modification of the bar 
windings just described. 

We have briefly reviewed here the armature windings at 
present in general use and may now pass to the various wind- 
ings employed for the field magnets. These are, in continuous 
current dynamos, almost always connected with, and supplied 
with current from, the armature winding, thus making the 
machines self -exciting. As the armature is turned the action 
begins with the weak residual magnetism left in the field mag- 
nets, and the current set up by the small E. M. F, thus produced 
is passed around and gradually strengthens the magnets, build- 
ing them up to full strength. If this residual magnetism is 
very feeble, as may happen when it is knocked out of the iron 



84 



ELECTRIC TRANSMISSION OF POWER. 



by rough handling or the continual jarring of a long journey, 
it is sometimes quite difficult to get the machine into action. 

The simplest form of field winding, and the one which was 
most extensively used at first, is that in which the current from 
one of the brushes passes around the field magnet coils on its 
way to or from the external circuit of the machine, as shown 
in Fig. 29. This series winding possesses more than one ad- 
vantage. It consists of a comparatively small number of con- 
volutions of rather large wire and so is cheap to wind, it is, 
for this same reason, little liable to injury and easy to repair 
when injured; and what is of particular importance, whenever 





Fig. 29. 



Fig. 30. 



the series dynamo is called upon for more current, the mag- 
netizing power of the field is raised by the increase, thus 
increasing the electromotive force. This property, once con- 
sidered a disadvantage, becomes of great value in modern 
windings adapted for the purpose. As the generation of 
E. M. F. at the start depends entirely on the residual mag- 
netism, series wound machines do not ''build up" full voltage 
very easily unless the resistance of the outside circuit is fairly 
low, thus giving the current a chance. 

The common shunt winding shown in Fig. 30 almost describes 
itself. The brushes are, independently of the circuit, connected 
to magnetizing coils of relatively fine wire. Although such a 



TRANSMISSION BY CONTINUOUS CURRENTS. 85 

field winding is slightly harder to construct and to maintain, it 
produces a magnetic field that is relatively free from any actions 
in the working circuit of the machine. So long as the E. M. F. 
at the brushes is unaffected by changes of speed, the field will 
be quite steady except as a very large current in the exterior 
circuit may reduce the voltage available for the field by causing 
a loss of voltage in the armature. If the armature resistance 
be very small, there will be almost a constant E. M. F. at the 
brushes except as the current flowing in the armature may 
produce a magnetization opposed to the shunt field. For a 
considerable time, then, the shunt winding was always used 
when a constant E. M. F. was required. At the same time, it 
permits the E. M. F. to be varied, if desired, with a very small 
loss of energy, by the simple expedient of putting a variable 
resistance in circuit with the field magnets. 

As the principles of dynamo construction became better 
laiown, it was apparent that the above method of getting a 
constant E. M. F. was rather expensive. To build an armature 
that v/ould carry a heavy current without noticeable loss of 
voltage and to inclose it in fields so strong as to be disturbed 
only in a minute degree by the magnetizing effects of such 
current, was a task requiring much care and a great amount of 
material. Even if this difficult problem were solved, the con- 
stant voltage would be at the brushes of the machine and not 
at the load, where it is needed. 

An easy way out of these difficulties is found by considering 
an important property of the series-wound machine just men- 
tioned, i.e., the rise of E. M. F. as the load on the external 
circuit rises. If now one takes a good shunt- wound dynamo 
and adds to the field magnets a few series turns wound in the 
same direction as the shunt, the result is as follows: At no 
load, the voltage at the brushes is that due to the shunt alone. 
As the load comes on this voltage would naturally fall off by the 
loss of voltage from armature resistance and reaction. The 
series turns, however, at this juncture strengthen the field and 
thus compensate for these losses. This is the compound wind- 
ing, now almost universally used. It is sho^vn in diagram in 
Fig. 31. Ordinarily the series turns- are more than would be 
needed for merely compensating the losses due to armature 



86 



ELECTRIC TRANSMISSION OF POWER. 



resistance and reaction, so that the voltage at the brushes 
under load will rise enough to make up for the increased loss 
in the line due to carrying heavier current. 

Machines thus over-compounded five or ten per cent are in 
very common use. 

The foregoing gives the rudiments of the machines used for 
generating direct current. It now remains, before taking up 
the question of power transmission proper, to consider briefly 
the use of such machines as motors. The underlying principle 
has been already discussed. The power of a motor to do 




Fig. 31. 



work depends on the stress of the magnetic field on conductors 
carrying curent in it and free to move. This stress is virtu- 
ally the same as that which has to be overcome in using the 
machine as a generator, and reaches a very considerable 
amount in machines of any size. 

In motors with the field strengths often used, the actual 
drag between the field and the armature wires may amount at a 
rough approximation, to nearly an ounce pull on each foot of 
conductor in the field for every ampere flowing through the 
wire. With a 20 HP motor the actual twisting effort or torque 
Sit the surface of the armature might easily be considerably 
over a hundred pounds pull. Forces of this size emphasize 



TRANSMISSION BY CONTINUOUS CURRENTS. 87 

the need of solid armature construction, with the conduc- 
tors firmly locked in place, particularly since the magnetic 
drag is not steady, but comes som.ewhat violently upon the 
conductors as they enter the field. With the old smooth core 
armatures wound with wire, the conductors not infrequently 
worked loose and chafed each other, and even the entire wind- 
ing has been known to slip on the core. In modern windings, 
either iron -clad or modified smooth core, such accidents are 
nearly impossible. 

When the armature conductors of the motor cut through its 
field as the armature revolves, an electromotive force is neces- 
sarily generated in them as in every other case when the 
magnetic forces on a conductor change. There is thus pro- 
duced, as a necessary part of the action of every motor, a 
counter electromotive force in the armature. This electro- 
motive force plays a very important part in the internal 
economy of the motor. 

In the first place, the magnitude of the counter electromo- 
tive force determines the amount of current that can flow 
through the motor when supplied at a given voltage. The 
resistance of the armature from brush to biush may be only 
a few thousandths or even ten thousandths of an ohm, 
while the applied voltage may be several hundred volts. The 
resulting current, however, is not that which would flow 
through the given resistance under the pressure applied, but 
the flow is determined by the difference between the applied 
electromotive force and the counter E. M. F. of the motor, so 
that in starting a motor when the armature is at rest and there 
is therefore no counter E. M. F., a resistance must be inserted 
outside the armature to cut down the initial rush of current. 

In the second place, the counter electromotive force mea- 
sures the output of the motor for any given current. It does 
this because the very same things, i.e., strength of field, 
amount of wire under induction, and speed, which determine 
the output for a given current, also determine the magnitude 
of the counter electromotive force. 

Therefore, when the machine is running as a motor, while 
the energy supplied to it is the product of the voltage by the 
ampere^ which flow tlirough the armature, the output of the 



88 ELECTRIC TRANSMISSION OF POWER. 

motor is determined by the product of the counter electromo- 
tive force into the selfsame current; hence, under given con-, 
ditions, the ratio between the counter and impressed electro- 
motive forces of the motor determines the efficiency of the 
motor. The difference between these electromotive forces 
determines the input of energy, since it determines the cur- 
rent which may flow; therefore, as the counter electromotive 
force increases, the efficiency of the motor increases, but the 
output is limited by the decreased input. 

With a fixed electromotive force supplied to the armature, 
the output of the motor per ampere of current will diminish as 
the counter electromotive force diminishes, but the total 
amperes flowing will increase because the difference between 
the appfied and counter E. M. F. has also increased. Thus 
the total output increases, although at a lower efficiency, 
when the counter E. M. F. decreases. Since the input (which 
is determined by the difference between counter and applied 
E .M. F.'s) multipfied by the efficiency (which is determined 
by the counter E. M. F.) equals the net output of the motor, 
this output will be at a maximum when the counter E. M. F. 
and the effective E. M. F. are equal to each other. This fol- 
lows from the general law, that the product of two quantities, 
the sum of which is fixed, will be a maximum when these 
quantities are equal. 

It must be distinctly understood, however, that at this point 
of theoretical maximum output the motor is very inefficient, 
and that mechanical considerations prevent the efficiency 
being wholly determined by the counter E. M. F., while spark- 
ing and heating generally prevent working with the counter 
E. M. F. equal to the effective E. M. F. 

In actual practice motors are worked under very diverse 
conditions, and some of these it is worth while to take up in 
detail, following the preceding generalizations. The energy 
may be supplied at constant current, at constant voltage, with 
neither current nor voltage constant, at fixed or variable 
speed, and subject to a wide variety of conditions; the motors 
may be wound either series, shunt, compound, or with various 
modifications of these windings, and may be either self regu- 
lating with respect to various requirements, or regulated by 



TRANSMISSION BY CONTINUOUS CURRENTS. 89 

extraneous means. In the ordinary problems dealt with in 
power transmission, these conditions may be classified in a 
fairly simple way as follows : 

Case I. Series- wound motors at constant current. 

Case II. Series-wound motors at constant voltage. 

Case III. Series- wound motors with interdependent current 
and voltage. 

Case IV. Shunt-wound motors at constant voltage. 

The first class is now rarely found in practice, and is of 
real commercial importance only in a few cases. The second 
class is very widely used in a particular case, to wit: electric 
railway service, and consequently it is of great practical im- 
portance. The third class of motors is used occasionally with 
great success but not very extensively, while the fourth includes 
the vast majority of all the continuous current machines 
running for purposes other than electric railway service. 
These latter cases, therefore, it is worth while to take up 
somewhat thoroughly. 

Case I. — Series-wound motors operated with a constant cur- 
rent originally came into use in connection with arc lighting 
circuits, which for some years formed the most generally 
available source of current. Such lines are fed from dynamos 
in which the current is kept constant by special regulation, 
while the voltage rises and falls in accordance with the load, 
consisting of lamps or motors in series with each other. We 
are therefore relieved of any concern about the current, since 
it is kept constant quite irrespective of what happens in the 
motor. 

Under these circumstances, in a series-wound motor, the 
torque will be constant, since the field is constant, and the 
output of the motor will vary directly with the speed. If it 
be loaded beyond its capacity, it simply refuses to start the 
load, inasmuch as its torque is limited by the current. If it 
starts with a load within its limit of torque, its speed will 
steadily increase until that limit is reached. This may be 
comparatively soon if the load is a rapidly increasing one, or 
the machine may race until its own friction of air and bearings, 
magnetic resistances, and the induction of idle currents in the 
core and frame serve to furnish resistance up to its limit of 



90 



ELECTRIC TRANSMISSION OF POWER. 



torque. When running at a given speed, any increase of load 
causes the speed to fall off, while decrease of load produces 
racing. Unless these tendencies are controlled, this type of 
machine becomes almost useless for practical purposes, as 
regularity of speed under change of load is generally highly 
desirable. In fact, the tendency to run at constant torque is 
generally inconvenient. To obviate this very serious difficulty, 
various devices have been tried with tolerable success. The 
commonest is to vary the torque in accordance with the load 
by changing the field strength, or by shifting the brushes so as 




to throw the armature coils out of their normal relation to the 
magnetic field. 

Since the object of such changes is to vary the output at 
constant current, and since this output is measured by the 
counter E. M. F. of the motor, the real problem of such regu- 
lation is to vary the counter E. M. F. in proportion to the 
output desired. Therefore, the same general means that 
serve to accomplish this end in an arc dynamo, keeping the 
current constant and varying the E. M. F., will serve to regu- 
late the corresponding motor. 

As in this case the speed is the thing to be held constant, 
the usual means taken for working the regulating devices is a 



TRANSMISSION BY CONTINUOUS CURRENTS. 91 

centrifugal governor, which generally acts to shift the brushes 
or to put in circuit more or less of the field winding, which for 
this purpose is divided into sections. In still other arrange- 
ments the governor acts to slide the armature partially into or 
out of the field, or to work a rheostat which shunts the field 
magnet, as in the Brush regulator for constant current. An 
excellent example of a small constant current motor regulated 
on the last mentioned principle is shown in Fig. 32. 

As to the operation of these regulating devices, it is tolerably 
good if everything is carefully looked after and kept in adjust- 
ment. The efficiency of such motors is not generally as high 
as that of other types at light loads, owing to the nearly con- 
stant loss in the armature due to constant current working. 
At and near full load the efficiency may be good. 

In addition, the current is highly dangerous, coming as it 
does from generators of very high voltage, and even the voltage 
across the brushes is, in machines of any size, sufficient to 
give a dangerous or even fatal shock. A 10 HP motor, for 
example, on the customary 10-ampere circuit, would have a 
difference of potential of about 800 volts between the brushes 
at full load. As a few such motors would load even the largest 
arc dynamos, besides being dangerous in themselves, opera- 
tions have generally been confined to smaller units. On 
account of the danger and the mechanical and other difficulties, 
the arc motor has come to be looked upon as a last resort, is 
seldom or never used when anything else is available, and, to 
the credit of the various manufacturers be it said, is nearly 
always sold and installed with a specific explanation of its 
general character and the precautions that must be taken 
with it. 

In spite of all these objections, the constant current motor 
often does good and steady work, and some such motors have 
been used for years without accident or serious trouble of any 
kind. They have been employed, however, only sparingly for 
power transmission work of any kind, and when so used are 
mostly on special circuits of 50 to 150 amperes. 

Case II. — Series motors worked at constant potential are 
very widely used for electric railway service and other cases, 
such as hoisting, in which great variations of both speed and 



92 ELECTRIC TRANSMISSION OF POWER. 

torque are desirable. When supplied at constant potential, the 
speed of a series-wound motor varies widely with the load. In 
any case the speed increases until the counter E. M. F. rises 
high enough to cut the current down to the amount necessary 
to give the torque sufficient for that load and speed. 

If the field be strengthened, the motor will give a certain 
output at a lower speed than before; if it be weakened, at a 
higher speed; the torque being in these cases correspondingly 
increased or decreased. 

The torque increases rapidly with the current, so that when 
the counter E. M. F. is small, or zero, as in starting from rest, 
the torque is very great, a property of immense value in start- 
ing heavy loads. For in starting, not only is the current 
through the armature large, but the field is at its maximum 
strength. If the field strength varied directly as the current, 
the torque w^ould vary nearly as the square of the current. 

As a rule, however, these, like most other motors, are worked 
with a fairly intense magnetization of the fields, so that doub- 
ling the magnetizing current by no means doubles the strength 
of the field. In fact, most series motors for constant potential 
circuits are of the type used for electric railways, and wound 
so that the field magnets are nearly saturated even with very 
moderate currents. Hence the torque in such cases increases 
but a trifle faster than the current. This construction is 
adopted in order to reduce the amount of iron necessary to 
secure a given strength of field, and so to lighten and cheapen 
the motor. 

It is quite obvious that while series motors at constant 
potential have the advantage of being able to give on occasion 
very great torque, they suffer from the same disadvantage' as 
constant current motors, in that they are not self-regulating 
for constant speed. Centrifugal governors could, of course, 
be applied to them, but since it happens that most work 
requiring great torque also requires variable speed, nothing 
of the kind is usually necessary. 

As previously explained the speed can be easily regulated to 
a certain extent by changing the field strength, thus changing 
the counter E. M. F., but owing to the peculiarity of design 
just noted, this method is rather ineffective, requiring a 



TRANSMISSION BY CONTINUOUS CURRENTS. 



93 



great change in the field winding for a moderate change in 
speed. 

In general, when a considerable range of speed is needed, 
constant potential working is abandoned and the speed is 
changed by varying the impressed E. M. F. by means of a 
rheostat. If this E. M. F. be lowered, the current decreases 
and the speed sags off until the new counter E. M. F. is low 
enough to let pass just enough current to maintain the output 
at the reduced speed. When the applied E. M. F. is increased 
the reverse action takes place. Under these circumstances, for 




Fig. 33. 



a fixed load the current is approximately the same, independent 
of the speed; for with a uniform load the torque is constant, 
while the output {i.e., rate of driving the load) varies. Many 
railway motors are regulated in the manner just described, 
although in addition the field strength is sometimes varied by 
cutting out or recombining fields and by series parallel control. 
Rheostatic control necessarily wastes energy, and the greatest 
recent improvement in railway practice consists in reducing 
the E. M. F. applied to the car motors by throwing them in 
series. This secures a low speed economically, though the 
rheostat still comes into play at intermediate speeds. 



94 ELECTRIC TRANSMISSION OF POWER. 

Speaking broadly then, series-wound motors, while possess- 
ing many valuable properties, are limited in their usefulness 
by their tendency to vary widely in speed when the load 
changes. Hence they are used chiefly in cases where the 
speed is to be varied deliberately. A typical early motor of 
this class, used for hoists and the like, with rheostatic control, 
is shown in Fig. 33. 

In spite of the difficulty in regulation, the series motor pos- 
sesses some considerable advantages: The field coils being of 
coarse wire are easily and cheaply wound, even in motors for 
very high voltage; the same quick response to changes in 
current or load that makes it hard to obtain uniform speed is 
also most important in many kinds of work; the powerful 
initial torque is coupled with the useful property of prompt 
reversal. All these make the series motor preeminent for cer- 
tain purposes, especially where severe work is to be coupled 
with hard usage. 

There is one case, too, in which the series-wound motor can 
be made accurately self -regulating for constant speed — a case 
somewhat peculiar and unusual, but yet worthy of special 
attention. 

Case III. — We have seen that when the load on a series 
motor supphed at a certain voltage increases, the speed falls 
off until the increasing current due to the lessened counter 
E. M. F. raises the torque sufficiently to meet the new con- 
ditions. Imagine now the impressed E. M. F. to be so varied 
that the slightest increase of current in the motor is met by 
a rise in the E. M. F. applied to it. Evidently the speed 
would not have to fall as before, for the greater applied voltage 
would furnish ample current for all the needs of the load. If 
the variation in voltage could be made to depend on change 
of torque, not giving the speed time to change, the regulation 
would be almost perfect. Such a method has been proposed, 
but owing to mechanical difficulties has not been used to any 
extent. 

It is possible, however, so to combine a special motor and 
generator that the former will be very closely uniform in speed, 
quite independent of the load. In this connection we must 
revert to the properties of the series-wound dynamo. If such 



TRANSMISSION BY CONTINUOUS CURRENTS. 



95 



a machine be driven at constant speed its electromotive force 
will increase with the current, since the strength of field, 
here the only variable factor in the voltage, will increase with 
the current. If the field magnets of the generator are 
unsaturated, that is, not so strongly magnetized as to require 
considerable current to produce a moderate increase of mag- 
netization, they will respond very promptly to an increase of 
load by raising the voltage. If such a generator be connected 
to a series- wound motor of proper design, the pair will work 
together almost as if connected by a belt instead of a long 
line, and the motor will run at a nearly uniform speed, since 




Fig. 34. 



the least diminution of speed, with its accompanying increase 
of current, will be met by a rise in the voltage of the genera- 
tor. Such an arrangement is shown in diagram in Fig. 34. 

In this cut A is the generator supplying current to the 
motor B. The machines should be of practically the same 
capacity, for the generator cannot supply current except to the 
one motor without disturbing the regulation. Whenever the 
load on B changes, a very small reduction in speed suffices to 
raise the voltage of A and thereby to hold up the speed of B. 
To this end the field magnets of B must be more strongly 
saturated than those of A, else the same increase of current 
will raise the counter E. M. F. of the motor and defeat the 
purpose of the combination. If the fields of the two machines 



96 ELECTRIC TRANSMISSION OF POWER. 

are properly designed, the generator will increase its voltage 
under increasing load just enough to hold the motor at speed, 
as a very slight change in current immediately reacts on the 
generator. 

It is even possible to make the motor rise in speed under 
load if the generator is sufficiently sensitive to changes of cur- 
rent. This is generally needless, but it is often useful so to 
design A and B that the former will rise in voltage fast enough 
not only to compensate for the added load on the motor but 
for the added loss of energy in the line, entailed by the in- 
crease of current, thus regulating the motor even at a long 
distance. The difference of saturation between the generator 
and motor fields need not involve material difference of design, 
since it may be effected by shunting the motor field. When 
properly adjusted, the system is capable of holding the motor 
speed constant within two per cent through the range of load 
for which the machines are planned. 

It should be noted in connection with Fig. 34 that, whereas 
the current circulating in the armature of a generator tends to 
disturb the magnetic field in one direction, in a motor the same 
reaction is in the opposite direction. For the current in the 
motor is driven through the armature against the counter 
E. M. F., i.e., in the direction opposite to that of the current 
the machine would give if running as a generator. As the 
effect of the reaction is to skew the direction of the m.agnetic 
field that affects the armature conductors, and the commuta- 
tion must take place when the commuted coil is not under a 
varying induction, the armature reaction compels one to shift 
the brushes slightly away from the position they would have if 
the field were perfectly symmetrical. This shifting is in the 
direction of armature rotation in a generator, but for the 
reason above noted has the opposite direction in a motor. In 
either case it need be only a few degrees. 

Case IV. — Shunt- wound motors are almost invariably 
worked on constant potential circuits, to which they are partic- 
ularly well suited. They form by far the largest class of motors 
in general use and owe this advantage mainly to their beautiful 
self-regulating properties. 

The shunt motor is in construction practically the same as 



TRANSMISSION BY CONTINUOUS CURRENTS. 97 

a shunt-wound dynamo. Let us look into the action of such a 
machine when suppUed from a source of constant voltage. If 
the design be reasonably efficient, the armature will have a 
very low resistance and the shunt circuit, which includes the 
field coils, a resistance several hundred times greater. When 
such a machine is supplied with current of constant voltage at 
its brushes and is running at any given speed and load, the 
current through the armature is practically determined by the 
counter E. M. F. developed, the armature resistance being 
almost negligible. The shunt is of high resistance and takes 
a certain small amount of current, determined by the voltage 
across the brushes. Now let the load increase; the field is, 
aside from loss of voltage on the line, practically constant, and 
the first effect of the added load is, as in a series motor, to 
reduce the speed. But this lowers the counter E. M. F., and 
consequently the armature current rises and the torque is 
increased, thereby enabling the motor to operate under the 
larger load. The torque necessary to enable the motor to 
maintain an increased load varies directly as the load and is 
also directly proportional to the current. But since the cur- 
rent is closely proportional to the difference between the 
impressed and counter E. M. F.'s, it is possible to design a 
machine so as to run at almost exactly constant speed. 

The constancy depends really on the armature resistance, 
small as it is. For example, a motor is designed to run at 100 
volts. Running light the counter E. M. F. is 99.9 volts, and 
with an armature resistance of 0.01 ohm the current will be 10 
amperes. The work done is say 1 HP. Now let a full load, 
say 20 HP, be thrown on. The torque will have to be in- 
creased 20 times, requiring 200 amperes. But this will flow 
through the armature under an effective pressure of 2 volts. 
Hence the counter E. M. F. will only have to fall to 98 volts to 
provide current enough to meet the new condition. As the 
counter E. M. F. varies directly as the speed, a fall in speed of 
less than 2 per cent will follow the increase of load. This 
computation neglects all questions of armature reaction as 
well as the effect of this minute fall in speed on the output, 
but fairly represents a case that might actually be met with in 
the best modern practice. In fact, shunt motors have been so 



98 ELECTRIC TRANSMISSION OF POWER. 

designed as to vary no more than IJ per cent in speed from 
no load to full load. A variation of 5 or 6 per cent is, how- 
ever, more usual. 

When supplied from an over compounded generator so that 
the impressed voltage may increase with the load, a shunt 
motor can be operated even more closely to constant speed 
than indicated above, since there is no longer need for a fall 
in speed to maintain the requisite difference between the 
impressed and counter E. M. F.'s. In such case any tendency to 
fall in speed is at once corrected by the rise in voltage on the 
hne. This scheme is seldom used, however, since it is ill fitted 
for simultaneously operating a number of motors at varying 
loads, and for single units has no particular advantages over 
the series-wound pair previously noted, or a very simple 
arrangement of alternating apparatus. 

Not only can the shunt motor be worked at nearly constant 
speed, but it also has the advantage of permitting a consid- 
erable range of speed variation without sacrificing much in the 
matter of efficiency. We have already seen that a change in 
field strength involves a change of speed, since it necessarily 
alters the counter E. M. F., which in turn modifies the current. 

In a shunt motor the immediate effect of a decrease of field 
strength is to lower the counter E. M. F., letting more current 
through the armature and increasing the torque. Hence, the 
speed rises until the current and torque adjust themselves to 
the requirements of the load. On the other hand, if the field 
be strengthened, the current necessary to carry the load can- 
not be obtained without a fall in speed. It is clear that the 
changes of speed thus obtained may be quite considerable, for 
in a motor such as that just described a variation of 10 per 
cent in the field would produce an immense variation of cur- 
rent, which would have to be compensated by a change in 
speed as great as the change in the field. Inasmuch as these 
field changes can be produced by varying the field current, 
which is always small, through a rheostat in the circuit, this 
change of field strength can be accomplished with but a tri- 
fling waste of energy. If the field magnets are comparatively 
unsaturated, it is not difficult to obtain perhaps 50 per cent 
variation in speed. A motor designed for such work is, how- 



TRANSMISSION BY CONTINUOUS CURRENTS. 



99 



ever, bulky, as it must if necessary be possible to get torque 
enough to handle the full load with a field much below its 
normal strength. 

It should be noted that even when running at a considerably 
modified speed, the motor must still be nearly self-regulating 
for changes in load, for the conditions that govern self-regu- 
lation are within moderate limits unaffected by the particular 
strength of field employed. Only when the armature reaction 
has been greatly modified will the regulation be sensibly 
disturbed. 




Fig. 35. 



A device sometimes used to improve the regulation of motors 
essentially shunt wound is the so-called differential winding. 
This consists of an additional field winding in series with the 
armature, but around which the current flows in such a direc- 
tion as to demagnetize the field. The total field strength is 
then due to the difference between the magnetizing power of 
the shunt and of this regulating coil. When the load on the 
motor increases, the additional current due to a minute change 
of speed will weaken the field, and thence cause the motor to 
run faster until the counter E. M. F. adjusts the current to the 
new speed and output. Differential winding obviously requires 
an extra expenditure of energy in the field, since the shunt and 



100 



ELECTRIC TRANSMISSION OF POWER. 



series turns act against each other. Fig. 35 shows the Sprague 
motor wound on this differential plan, now only of historical 
interest, but which through its good qualities did much to 
popularize the electric motor in America. Plate II shows 
in Fig. 1 a Westinghouse bi-polar shunt motor, and in Fig. 
2 a G. E. six-pole shunt motor for slow speed. 

Various modifications of shunt- and series-wound motors 
have from time to time appeared, devised for particular adapta- 
tion to special purposes or sometimes merely for the sake of 
novelty. None of them, however, are of sufficiently general 
importance to find a place here except a single very beautiful 
method of obtaining efficiently a very wide range of speed. 

The principle of this method is to work the motor at normal 




Fig. 36. 



full excitation, but to deliver to the armature a current of 
variable E. M. F. so that a given current and hence torque 
may accompany very various values of the counter E. M. F. 
Fig. 36 shows the connections employed to effect this result. 
Here C is the working motor, B the special generator which 
feeds its armature, A the motor used to drive this generator, 
and D the rheostat and reversing switch in the generator field 
circuit which allows the generator E. M. F. to be varied. In 
the figure the motor H is shown as a synchronous alternating 
machine with a commutator from which are fed the fields of 
the three machines. In ordinary central station practice A is 
a continuous current motor, and the fields are fed direct from 
the distributing mains. The result of this arrangement is that 
the motor field C remains at full strength, while the armature 




Fig. 1. 




Fid. % 



PLATE II, 



TRAl^SMISSlOK^ BY CONTINUOUS CURRENTS. 101 

current can be brought to any required strength at any desired 
armature speed within a very wide range. Hence, C can give 
full load torque or even more while the armature is merely 
turning over a few times per minute, and the speed can be 
brought up with the utmost delicacy and held at any desired 
point. And at every speed the motor holds its speed fairly 
well, irrespective of changes of load. For elevators, hoists, 
and similar work this device is extremely useful. The only 
objection to it is the cost of installing the two extra machines, 
which is of course considerable. Nevertheless the regulation 
attained is so beautiful and perfect that the cost often becomes 
a minor consideration, and the device is very widely used in 
cases where variable speed is essential. 

POWER TRANSMISSION AT CONSTANT CURRENT. 

In its general aspect this method must now be regarded as a 
makeshift. It came into existence at a time when the only 
circuits extensively installed were those for arc lighting, and 
hence, if motors were to be used at all, they must needs be of 
the constant current type. As incandescent lighting became 
more common the arc motors were gradually replaced by shunt 
motors worked at constant potential. A few constant current 
plants especially for motor service have been installed both 
here and abroad, but for the most part they have merely 
dragged out a precarious existence, and in this country have 
been abandoned. 

There is good reason for this. The motors usually regulate 
indifferently, and there is serious objection to running high 
voltage wires into buildings when it can be avoided. 

The objections of the insurance companies alone are quite 
sufficient to discourage the practice. The constant current 
has often been advocated for long distance transmission of 
power where high voltage is a necessity. For such service the 
method has the great advantage that the motors do not need 
extraordinary insulation except from the ground. A constant 
potential service at 5,000 volts continuous current would be 
utterly impracticable, if distribution of power in moderate 
units were to be attempted, while with constant currents it 



102 



ELECTRIC TRANSMISSION OF POWER. 



is entirely feasible, although objectionable on the grounds 
mentioned. In addition, unless a proposed transmission be 
for power alone, the constant current method shares with con- 
stant potential of high voltage the very grave difficulty that 
an incandescent lamp service is out of the question, without 
secondary transformation of the necessarily high line voltage 
to a very moderate pressure. This is somewhat expensive with 
continuous currents of any kind, and at once introduces the 




Fig. 37. 



troublesome question of regulation at constant current into 
the problem. 

To reduce the energy sent over a high voltage continuous 
current line to a pressure at which incandescent lamps can be 
fed, two methods are possible. We may pass by the plan 
of using many lamps in series as of very limited appli- 
cability and forbidden by the fire underwriters. First, the 
required power may be received by a motor of appropriate 
size, which is belted or coupled to a low voltage generator. 
This device does the work, but it involves installing three 
times the capacity of the lamps desired in machinery of a 
somewhat costly character, and losing in the motor and gen- 



TRANSMISSION BY CONTINUOUS CURRENTS. 103 

erator perhaps 15 or 20 per cent of the energy supplied from 
the Une. The other alternative is to employ a composite 
machine combining the functions of motor and generator. 
This piece of apparatus is variously known as a motor-genera- 
tor, dynamotor, or continuous current converter. It is a 
dynamo electric machine having a double-wound armature and 
two commutators. One winding with its commutator receives 
the high line voltage and operates as a motor. The other 
winding, and its commutator furnishes, as a dynamo, low 
tension current. The field is common to both windings. Fig. 
37 shows a small machine of this kind, adapted to receive 5,000 
volts from the line, and to deliver 110 volts, or vice versa. 

This particular machine works at constant voltage on both 
circuits. Either circuit, however, could be made to work at 
constant current, provided the means of regulation for this 
purpose were so chosen as to leave the field and speed un- 
changed. 

The cost of a motor generator, while less than that of two 
separate machines, is still high, and although its efficiency is 
somewhat greater than that of the pair mentioned above, it is 
obtained at the cost of a rather complicated armature, which, 
from a practical standpoint, is quite objectionable. 

In spite of the difficulties incident to working incandescent 
lamps from a high voltage constant current circuit, the ease 
with which such circuits can be worked, even if for power 
alone, at voltages far above those available on the constant 
potential system, encouraged their installation during the 
period between the first efforts at long distance transmission 
and the more recent date at which alternating current appa- 
ratus has become thoroughly available. For some years it 
was constant current or nothing, so far as long distance trans- 
mission, coupled with distribution, was concerned. 

As a result of the various adverse conditions mentioned, 
transmission at constant current has never made really any 
headway in American practice, and in fact the method has been 
followed to a noticeable extent in only one locality — San Fran- 
cisco. There, through the activity of local exploiters, constant 
current power circuits were early established and remained 
in fairly successful operation for several years. 



104 ELECTRIC TRANSMISSION OF POWER. 

There were until recently three companies operating con- 
stant current circuits in San Francisco for the distribution 
of power. The currents empoyed were of 10, 15, and 20 am- 
peres. Most of the motors were small, a very large propor- 
tion of them being under one horse-power. The total num- 
ber of motors in circuit on the various systems was between 
six and seven hundred. 

Except in San Francisco, what few constant current motors 
have been in operation were operated on regular arc circuits. 
Their use has been much discouraged by the operating com- 
panies, and very few such motors are now manufactured or 
sold ; in fact, constant current distribution in modern American 
practice is almost non-existent. Abroad, the conditions are 
somewhat different, and on the Continent constant current 
distribution for long distance transmission work has been ex- 
ploited to a very considerable extent, probably owing to the 
early and successful establishment of a number of transmission 
plants for single motors worked on the series system. There 
are several successful plants operated at constant current, one 
of the most considerable of them being that at Genoa, which 
is an excellent example of the kind and as such is worth more 
than a passing mention, even although the probability is that it 
will seldom be duplicated, at least on this side of the Atlantic. 

The Genoa transmission is derived from the River Gorzente, 
which about twenty years ago was developed for hydraulic pur- 
poses, artificial lakes being established and a tunnel about 1^ 
miles long being built for an outlet. Beyond the tunnel, an 
aqueduct some fifteen miles in length conveyed the water to 
Genoa, where a considerable amount of power is utilized 
directly. In this development there was left at the mouth of 
the tunnel an unused fall of nearly 1,200 feet aside from the 
head employed in the aqueduct. This has been developed 
electrically. It was divided into three partial falls of 338,357, 
and 488 feet, respectively. At each of these was erected a 
generating station with its own transmission line. These 
stations were named after the three renowned electricians, 
Galvani, Volta, and Pacinotti. The first mentioned station 
was the first installed. It consists of two generators operated 
in series. Each is of about 50 KW capacity, giving 47 amperes 



TRANSMISSION BY CONTINUOUS CURRENTS. 105 




106 ELECTRIC TRANSMISSION OF POWER. 

with a maximum pressure of 1^00 volts. Current is kept con- 
stant by regulating by hand the speed of the dynamos, 
through the gate which controls the turbines. Each dynamo 
is provided with an automatic switch, short circuiting the ma- 
chine in case of extreme rise in voltage. This Galvani station 
was a preliminary or experimental station, and was followed up 
by the establishment of two others which supply the power to 
Genoa. One of these stations, which is thoroughly typical of 
the system^ employed, is shown in Fig. 38. It consists of four 
turbines, each driving a pair of dynamos of a little less than 
50 KW output at 45 amperes and about 1,000 volts. 

These dynamos are similar to those in the Galvani station, 
but the regulation for constant current is obtained in a dif- 
ferent manner. The dynamos are separately excited, the 
fields being supplied in parallel from a small dynamo driven 
by a separate waterwheel. The speed of this exciter is auto- 
matically varied by controlling its turbine in response to 
changes of current in the circuit. All the dynamos are oper- 
ated in series, and like those in the Galvani station are direct 
coupled in pairs. The machines are insulated with enormous 
care, heavy layers of mica being placed between the magnets 
and the bed plates, while the windings themselves are very 
elaborately protected. Carbon brushes are employed, and 
the commutators are reported to behave admirably. Each 
dynamo is protected by most elaborate safety devices, as in the 
Galvani station. The regulation is said to be excellent, even 
under considerable changes of load. 

The third station, Pacinotti, contains eight machines of the 
same capacity as those in the preceding. They are governed 
as in the Galvani station by controlling their speed. This, 
however, is done by an electrical motor-governor controlled 
by a relay on the main line and working in one direction or 
the other as the occasion may require. In all the stations are 
carried out the same thorough precautions regarding insula- 
tion, and each machine has around it an insulated floor sup- 
ported on porcelain. The line voltage from each of the two 
stations last mentioned is from 6,000 to 8,000, and two 
circuits are carried into Genoa, the extreme distance of trans- 
mission being about eighteen miles. The conductors, of wire 



TRANSMISSION BY CONTINUOUS CURRENTS. 107 

9 mm. in diameter (nearly No. 00 B. & S.), are for the most 
part bare, except when passing through villages, and are sup- 
ported on oil insulators carried by wooden poles, save in some 
few cases where iron poles have been used. The two circuits 
running into Genoa transmit power for motors only. The loss 
in the line at full load is 8 per cent. 

At full load, nearly 1,000 HP is transmitted over the lines, 
the motors being of all sizes between 10 and 120 horse-power. 
They are of the ordinary series-wound type, and their speed is 
automatically controlled by centrifugal governors, which act 
by varying the field strength. Fig. 39 shows one of the Genoa 
motors, fitted with an automatic governor acting upon a com- 



FlG. 



mutated field winding. They are provided with carbon 
brushes, and are reported to operate very successfully. 

It is to be noted, however, that the motors are placed in 
special rooms with insulated floors and walls, owing to the 
enormous voltage which has to be taken into the buildings. 
They are fitted with heavy fly-wheels to assist the governors, 
and with automatic switches to short circuit around the motor 
in case of excessive voltage. The motors are under the 
special care of skilled assistants connected with the staff of 
the generating station, who inspect the lines and go over the 
motors at intervals of a few days. These extraordinary pre- 
cautions both in the matter of insulation and skilled attendance 
account in great measure for the success of what, under 



108 ELECTRIC TRANSMISSION OF POWER. 

American conditions, would have almost infallibly resulted in 
disastrous failure. The efficiency of the plant from turbine 
shaft to motor pulleys is said to be a little over 70 per cent. 

As may be judged from this description, the whole instal- 
lation is of enormously complicated character, although per- 
haps as simple and efficient as any alternating plant of the 
same early date. The plan of the Volta station for the most 
part explains .itself. The switchboards for each machine with 
their plugs for connecting the pair of dynamos coupled to it are 
shown at A, dynamos at B, the exciters at C, exciter switch- 
board and rheostat at D, and the solenoids, which control the 
exciter turbines, at E. Lightning arresters are shown at F. 
These consist of a spark gap, impedance coils in series with 
the line and condenser shunted around them. Every motor is 
provided with a similar lightning arrester. Taken altogether, 
this Genoa plant is an excellent example of the constant cur- 
rent system followed to its legitimate conclusion. A descrip- 
tion of the system is a sufficiently condemnatory criticism 
judged from our present point of view; at the same time, it 
should be remembered that while this station was being built, 
the method adopted was practically as good as any available 
in the existing state of the art, and that the system has in 
more recent installations been materially improved. En- 
couraged by the favorable results obtained at Genoa, a similar 
station was soon afterwards built delivering a maximum of 
700 HP at Brescia at an extreme full-load pressure of about 
15,000 volts over a 12-mile line. Since these stations nearly 
a dozen others have been installed, aggregating about 17,000 
HP, and their performance has been uniformly good. In 
spite of a predilection for modern polyphase work one must 
admit that a system which has been installed to such an 
extent, and of late in competition with alternating methods, 
is far from moribund. Two strong points it undoubtedly has; 
freedom from all inductive disturbances, and the property of 
carrying its extreme voltage only at full load, the importance 
of which will be discussed later. It has shown itself capable 
of doing steady and efficient work over long distances and 
under climatic conditions by no means favorable. The Con- 
tinental makers of this class of machinery have gone far be- 



r 



§L.^ 




PLATE III. 



TRANSMISSION BY CONTINUOUS CURRENTS. 109 

yond anything that has been attempted in American practice 
and have turned out constant current dynamos of really 
remarkable properties. 

At present, machines of 50 to 60 amperes have been given 
successfully E. M. F. as high as 3,500 volts, while those of 
100 to 150 amperes have gone to 2,500 volts. As they are 
usually coupled in pairs, a single unit may have a capacity of 
about 700 KW, each component machine giving over 300. 
Without pushing beyond present apparatus it then becomes 
possible to arrange a plant of 1,000 to 1,500 KW having a 
working E. M. F. at full load of 10,000 to 14,000 volts. Such 
a plant is not especially complicated and is nearly as easy to 
operate as an alternating plant. For a load of a few large 
motors, it is capable of good work, without, however, present- 
ing any advantages over a polyphase system save that the 
line is simpler and the insulation requirements less severe. 
An alternating power station of similar output would contain 
practically as many generators, for sake of security. When 
it comes to combined lighting and power service the constant 
current system is hard pushed. In practice, recourse is had 
to motor generators. Perhaps the best idea of the situation 
may be given by a brief description of the Swiss transmission 
from Combe-Garot to La Chaux-de-Fonds, a distance of 32 
miles. At the former place are installed 8 generating units 
each giving 150 amperes at 1,800 volts, giving a total capacity 
of 2,160 KW at 14,400 volts. These generators are six pole 
Thury machines with drum armatures, and are series wound. 
Regulation is by automatic variation of the speed of the tur- 
bines, the normal full load speed being 300 r. p. m. The line 
is overhead, of cables having a cross section of about 300,000 
cm., bare except in the towns where the power is delivered — 
Locle, and I^a Chaux-de-Fonds at the end of the line. Motors 
aggregating 2,400 HP are in circuit at these points, 2,000 HP 
being used in the transforming stations. All motors above 20 
HP are upon the high tension circuit. The substation at La 
Chaux-de-Fonds is typical of the methods employed. It is 
equipped with motor-generators of 200 HP working a three 
wire system at 320 volts between the outside wires. One of 
the motor-generator units is shown in Plate III. It is composed 



110 



ELECTRIC TRANSMISSION OF POWER. 



of two six pole machines with a fly-wheel between them. The 
machine to the left is the motor and upon it is momited the 
automatic speed regulator. The principle upon which this 
works is shown in Fig. 40. The regulating shunt around the 
fields and the brush shifting mechanism are simultaneously 
actuated by the dogs thrown into gear by the governor. This 
form of governor is very generally used for the motors upon the 
system. 

The efficiency of both generator and motor under test has 




Fig. 40. 



been shown to be 93.5 per cent, or 87 per cent for the com- 
plete unit. Similar motor-generators in connection with a 
storage battery furnish current at 550 volts for railway service. 
Now the drop in the line at full load is 6 per cent, so that 
we are in position to make a very close estimate of the effi- 
ciency of the system from waterwheel to low tension mains. 
It is obviously 

93.5 X .94 X .87 - 76.5 per cent. 

This is a very creditable figure for the total efficiency, and it is 
worth while comparing it with the results ordinarily reached in 
polyphase working. Taking the generator at 94 per cent, the 
raising and reducing transformers at 97.5 per cent each, the 



TRANSMISSION BY CONTINUOUS CURRENTS. Ill 

line at .94, and the distributing banks of transformers at 96.5, 
we have 

.94 X .94 X .975 X .975 X .965 = .81. 

The difference is substantially that due to the difference in 
efhciency between the static transformers and the motor gen- 
erator. If the comparison be made with the railway part of 
the proposition, assuming the use of a rotary converter, the 
case would stand about as follows: 

.94 X .94 X .975 X .975 x .94 = .79. 

In the simple case of large motors the advantage lies rather 
the other way, for the constant current plant would show 

.935 X .94 X .935 = 82.1 

against, for the alternating plant, 

.94 X .94 X .975 X .975 x .94 = .79. 

This merely indicates that after passing the voltages which 
can be derived directly from the armature, more is lost in the 
transformers than is gained in a low voltage winding. The 
figures for the efficiency of the alternating plant are taken 
from actual data on machines of about the capacity concerned. 
To tell the unvarnished truth, a constant current transmission 
coupled with a three wire distribution at 220 to 250 volts on 
a side is capable of giving even the best alternating system a 
pretty hard rub over moderate distances. In this country 
no constant current power transmission machinery is avail- 
able, but where it is readily to be had, it is by no means out of 
the game. A still larger constant current transmission plant 
is now in operation between the falls of the Rhone at Saint- 
Maurice, and Lausanne, a distance of 34.8 miles. The first 
installation is of five pairs of generators aggregating 3,300 
KW, giving at full load a combined voltage of 23,000. The 
current in this case is 150 amperes, as in the Combe-Garot 
plant just described. 

POWER TRANSMISSION AT CONSTANT POTENTIAL. 

The transmission of power to series-wound motors at con- 
stant potential is a branch of the art which as regards station- 



112 ELECTRIC TRANSMISSION OF POWER. 

ary motors has been developed only in special cases. It is, 
however, the method universally employed for electric rail- 
way work. Two or three sporadic efforts have been made to 
operate electric railway systems at constant current, but with 
such indifferent success that the method has been abandoned. 
Counting in electric railways, it is safe to say that at present 
the majority of all electrical power transmission in the world 
is done mth series motors worked at constant potential or as 
nearly constant potential as may be practicable. As before 
mentioned, regulation is generally obtained through the use of 
rheostats in series with the motors, thereby cutting down the 
applied voltage, or by throwing the motors either in parallel 
or in series with each other, or in the third place by a combi- 
nation of the above methods. Concerning the operation of 
motors thus arranged, sufficient has been said to explain the 
situation clearly. The general good properties of the method 
are most prominently exhibited in the simplicity of the motor 
windings and the very powerful effort which can be obtained 
in starting the motors from rest. These properties are of 
extreme value in railway service. 

Aside from the operation of electric railways, series motors 
at constant potential are frequently employed for hoists and 
similar work where a powerful starting torque and considerable 
range of speed at the will of the operator are desirable. In 
spite of the large use of motors for such purposes, there are no 
plants either here or abroad which may be said to be operated 
exclusively after this method, for it is generally found desir- 
able to combine in the same system series-wound motors for 
severe work and shunt-wound motors for purposes where uni- 
form speed is of prime importance. As a rule the power trans- 
mission so accomplished is over a comparatively small distance 
and really involves the problem of distribution more than 
transmission alone. A very large number of electric hoists 
designed by different makers are in use at various points 
throughout this and other countries, doing service in mines, 
operating elevators of one kind or another, working derricks, and 
travelling cranes and employed for a large variety of similar 
purposes. Many of the motors employed are of the ordinary 
railway type. 



TRANSMISSION BY CONTINUOUS CURRENTS. 113 

The voltage utilized for this work, in America at least, is 
usually either 200 to 250 volts or 500 to 600 volts, the former 
being most generally used in mines, where difficulties of insu- 
lation are considerable, or in operating motors supplied by 
three- wire systems already installed. The latter voltage is gen- 
erally selected for work above ground. None of the plants so 
equipped are, however, sufficiently large or characteristic to be 
worth a detailed description. The power installations and the 
methods of distribution are in general, closely similar to those 
employed for electric railway work. Plants of higher vol- 
tage than from 500 to 600 are so infrequent as to be hardly 
worth considering in practical engineering. It is perfectly 
possible to wind series motors for voltages considerably ex- 
ceeding this figure, say for 1,000 or 1,200 volts, or, in rare 
cases, more, pro^dded the units arc of tolerable size, but inas- 
much as most plants for the distribution of power require both 
large and small motors, wound both series and shunt, the 
general voltage is in nearly every case kept at a point at which 
it will be easy to meet these varied requirements; therefore 
500 volts, the American standard for railway practice, has 
usually been selected. 

The only noteworthy exception may be found in the use 
of the Edison three-wire system for distribution of power 
to railway and other motors. By this method it become^ 
possible to transmit the power at the virtual voltage of 1,000 
and to employ 1,000 volt motors, either series- or shunt- wound 
for the larger units in order to help in preserving the general 
balance, while at the same time using motors of all sizes with 
any kind of direct current winding, connected between the 
middle wire and either of the outside wires. The advantages 
of such an arrangement are very evident, and if the number 
of motors be considerable, so that it is possible to balance the 
system with a fair degree of accuracy, we have at our disposal 
a very convenient method for the distribution of continuous 
currents. It is interesting to note that this scheme found its 
first considerable development in electric railway service itself. 
Of course, the use of both 110 and 220 volt motors on Edison 
three-wire systems is very common, but the extension of the 
plan to operating electric roads, and under conditions which 



114 



ELECTRIC TRANSMISSION OF POWER. 



as regards balance are somewhat trying, is a considerable 
step toward an individual method. 

The method of working electric railways on the three-wire 
system is well shown in Fig. 41. Here the road is a double 
track one, to which the method is generally best suited. The 
ground, track, and supplementary wires serve as a neutral 
wire, both tracks being placed in parallel for this purpose 
and thoroughly bonded. On a double track road, the cars 
running on each side of the system will be substantially the 
same in number, and if the total number of cars be consider- 
able, a very fair balance can be obtained, although never as 
good as is customarily and necessarily used in an Edison three- 
wire system for lighting. In order to still further improve 
the balance of the system and prevent its being disturbed as 




Fig. 41. 



might otherwise occur by a blockade on one track at some 
point, it is better to make the trolley wire above each track 
consist of sections of alternate polarity and of convenient 
length, so that even in case of a blockade, stopping a con- 
siderable number of cars, the load would be removed almost 
equally from both sides of the system. 

Installed in this way, a railway system is operated at a virtual 
voltage of 1,000, and the saving of copper over the ordinary 
distribution at 500 volts is considerable, in spite of the inevitable 
lack of balance and the loss of the track as part of the main 
conducting system. ISTevertheless, the three-wire distribution 
for tramways has proved generally unsatisfactory on account of 
the complication of the overhead wiring and the difficulty of 
preserving balance, so that it has entirely disappeared from 



TRANSMISSION BY CONTINUOUS CURRENTS. 115 

American practice. A few years ago it was tried in Bangor, 
Maine ; Portland, Oregon ; and experimentally elsewhere, but 
was abandoned after no very long experience as troublesome and 
unreliable. It has found some use abroad, even in a few recent 
plants, and is reported to be successful in spite of the difficulties 
here mentioned. 

In several of these foreign plants the voltage from either lead 
to the neutral is 750 to 1000 volts, and each car has its motors 
coupled in pairs so as to form a self-contained 3-wire unit. In 
this form a very important saving in copper is effected; and 
there is a good chance for successful distribution of power over 
considerable lines. 



INTERDEPENDENT DYNAMOS AND MOTORS. 

Aside from the distribution of power for railway purposes, 
by far the most interesting kind of power transmission by 
continuous currents is that in which a special combination of 
two series machines is employed, giving a self-regulating 
system comprising a motor unit and a generator unit. This 
plan has been successfully used abroad, but has never been 
employed in American engineering practice except in an 
experimental and tentative way, owing largely to the difficulties 
that have been encountered in the production of large direct- 
current generators for high voltage. , 

While it is not at all a difficult matter to build a machine 
giving five or six thousand volts with a rather small current, 
such as is used in arc lighting, the troubles at the commuta- 
tor have proved forbidding w^hen any attempt has been made 
to use currents large enough to obtain units of any consider- 
able size. Power transmission in this country took its first 
stimulus from the development of polyphase apparatus and 
methods, and consequently, so far as the art has now been 
carried forward, it has been almost wholly in the line of alter- 
nating current work. It is obvious, nevertheless, that a 
system of power transmission such as we are considering, 
possesses great convenience where single units are to be operated 
over moderately long distances. In the first place, the indue- 



116 ELECTRIC TRANSMISSION OF POWER. 

tive difficulties familair with alternating currents are avoided. 
In the second place, the motors are self-starting under load, 
a condition that was not true of alternating machinery prior 
to the introduction of the polyphase system. Through the 
energy of several foreign engineers, notably Mr. C. E. L. Brown, 
much was done in power transmission by this method long 
before alternating current apparatus had been suitably devel- 
oped. The same difficulties were encountered abroad as 
here. It proved to be very difficult to build machines of suffi- 
cient voltage and of any considerable output. 

In this connection it is noteworthy that nearly all the plants 
of this character on the Continent have been installed at rela- 
tively low voltages, most of them less than 1,000, correspond- 
ing in general character to the American plants over similar 
distances worked at constant potential. In the very few 
instances where long distances have been attempted, the 
usual method has been to employ generators and motors per- 
manently connected together in series, on account of the 
impracticability of getting sufficient power in one unit at a 
very high voltage. This proceeding somewhat complicates 
the system. In addition, the generators and motors have to 
be especially designed for each other in order to secure regu- 
lation; which, of itself, is a considerable disadvantage. 

This last difficulty may be in part avoided by using a shunt 
around the field coils of the generator, thereby changing its 
regulation under variations of current. A similar device is 
widely used in this country in connection with compound- 
wound generators, where a shunt applied across the terminals 
of the series coils is used to regulate the compounding. In 
either case, the obvious result of such a shunt is to diminish 
the change in the field produced by a given increase in cur- 
rent. In this way the necessity for special machines can be 
partially obviated. The plants installed on this peculiar series 
plan have been uniformly successful, and permit of the con- 
venient transmission of moderate amounts of power over con- 
siderable distances. Such plants have even been employed 
in connection with motor-generators to supply a general 
distribution system, though evidently at a high cost for appa- 
ratus. 



TRANSMISSION BY CONTINUOUS CURRENTS. 117 

In order to distribute low tension currents from such a 
transmission system, it is necessary to employ either a motor, 
coupled to a dynamo, or a composite machine with a double 
winding, as in case of transmission at constant current. Either 
alternative involves the loss of energy substantially equiva- 
lent to that lost in two dynamo-electric machines of the 
capacities concerned. These losses arc necessarily much more 
serious than those in an alternating current transformer, as 
has already been seen. They are likely to amount to from 12 
to 15 per cent, so that quite aside from the efficiency of the 
generating dynamo and of the line, the price paid for the privi- 
lege of obtaining a low tension current is considerable. 

For the delivery of power alone, where motors in series 
coupled to appropriate generators can be used, the method 
is well fitted for use under certain circumstances and is closely 
approximate in efficiency to that which would be obtained 
by an alternating current transmission over the same dis- 
tance. It is worth mentioning that one of the longest of the 
early power transmissions was operated upon this now obsoles- 
cent system. 

CONSTANT POTENTIAL SYSTEMS. 

Shunt- or compound-wound generators used in connection 
with shunt-wound motors have found very extensive use 
in this country, working over inconsiderable distances. The 
very obvious advantage of such a system is that it permits the 
ready distribution of power as well as its easy transmission. 
If it becomes necessary to transmit power from one point to 
another, the chances are much more than even that at the 
distributing end of the line it will be desirable to utilize the 
power in a number of units of varying size. Such an arrange- 
ment bars out transmission from series dynamos unless upon 
the constant current system with its inherent difficulties of 
regulation, whereas with shunt- wound apparatus the problem 
is easy. It often happens, as previously mentioned, that at 
the receiving end both series and shunt motors are used, the 
former for hoisting and similar work, the latter for operation 
at constant speed. 



118 ELECTRIC TRANSMISSION OF POWER. 

The growth of the electric railway has encouraged the estab- 
lishment of such transmission plants, and their number is very 
considerable, scattered over all parts of the Union, not a few 
of them being in the mining regions of the Rocky Mountains 
and on the Pacific coast, as well as in various isolated plants 
through the rest of the country. In most of them, the dis- 
tances being moderate, an initial voltage of from 500 to 600 
has been employed; more rarely, voltages ranging from 1,000 
to 1,800. Plants of these latter voltages have now generally 
been replaced by polyphase machines. The efficiency of this 
method of transmission is about the same as that of the series 
method, just described, but with the advantage that the 
shunt motor supplied at constant potential can advantageously 
be distributed wherever the work is to be done, while with inter- 
dependent series units any distribution of power has to be ac- 
complished by means of shafting and belting or its equivalent. 

The net efficiency from generator to driven machine is likely 
to be rather better with the transmission at constant potential 
than in the case just discussed. The generators and motors 
are of nearly the same efficiency; the line at ordinary dis- 
tances is customarily worked at about the same pressure in 
both methods, but distribution by shafting is far less efficient 
at any but short distances than distribution of electric power 
by wire. The loss from the centre of distribution to individual 
motors will very seldom exceed 5 per cent, while the loss in 
equivalent shafting will seldom be less than 10 per cent, and 
more often 20 or more; in fact, it generally turns out upon 
investigation that so far as efficiency is concerned there is 
a noticeable saving in transmitting power electrically, even 
within the limits of a mill or large factory, over the results 
which can be obtained by the use of transmission by shafts 
and belts. In a large building where the power is to be widely 
distributed, it seldom happens that the loss in the shafting is 
less than 25 per cent. Anything in excess of this figure repre- 
sents remarkably good practice. With motors, 80 per cent 
efficiency, if the units are of tolerable size, can be reached 
without much difficulty, and there are comparatively few 
cases where the efficiency need fall lower than 75. In such a 
plant, installed some years since in a Belgian gun factory, 



TRANSMISSION BY CONTINUOUS CURRENTS. 119 

and described in the last chapter, the guaranteed efficiency 
was 76.6 per cent. As the efficiency of the dynamo was 
reckoned at but 90 per cent, the total efficiency would in 
practice be raised without difficulty to 78 or 79 per cent at full 
load. 

As regards efficiency in general, aside from the disadvan- 
tages previously mentioned in changing the voltage of direct 
current circuits, the efficiency of transmission by such currents 
is in itself as high as has ever been reached by other means. 
There is no material difference between the efficiency of direct 
and alternating current generators, nor between the efficiency 
of direct current motors and the polyphase motors, at least, 
among alternating motors. In these particulars, the direct 
current is able to hold its own against all comers, and in the 
cost of motors it has at present a material advantage. 

The most important recent improvement in d. c. motors is the 
so-called " interpole " construction, which effects such an auto- 
matic shifting of the neutral points of the armature as to ensure 
almost complete abolition of sparking even under great varia- 
tions of load and field strength. Fig. 42 shows diagrammatically 
the features of the interpole construction. Small intermediate 
pole pieces are provided bearing series windings which give a 
cross magnetization, opposing that created by the armature wind- 
ings, and thus prevent that displacement of the field which is a 
prolific source of sparking. The series turns form a species of 
compound winding which neutrahzes the distortion of the field. 
The general idea of providing a special field for this purpose is 
an old one, but it is only recently that designers have gone to' 
the length of providing extra pole pieces to effect the result. 

The device is most useful and checks sparking even under 
severe overloads and great reductions in field strength. It is 
particularly valuable in motors designed, as in industrial plants, 
to operate over a great range of speed, and has also been a^pplied 
extensively to railway motors. It can, of course, be made simi- 
larly useful in generators, and when skillfully applied, seems to 
be almost a panacea for field distortion. It has, of course, limi- 
tations, just as compound winding has, but for all that it meets 
very successfully many difficult conditions. The additional poles 
involve some extra cost in material and labor, not enough, how- 



1^0 



ELECTRIC TRANSMISSION OF POWER. 



ever, to be a serious matter in the class of work in which extreme 
variations in load and speed are necessary. 




Fig. 42. Connections of Interpole Motor. 

current is able to hold its own against all comers, and in the 
cost of motors it has at present a material advantage. 

As regards transmission of power over considerable dis- 
tances, a case has already been mentioned in which the result 
is as good as can reasonably be expected. Direct current, 
however, continually runs into the limitation of available vol- 
tage as soon as distribution is to be attempted. Where single 
or a few large motor units are to be used, consisting of either 
single machines or groups operated as a unit, the efficiency of 
the system is likely to be as high as that obtained from units 
of similar magnitude on alternating current systems. The 
only disadvantage of the direct current in point of efficiency 
in this particular case is that if the amount of power to be 
transmitted be large, it is necessary to use generators and 



TRANSMISSION BY CONTINUOUS CURBBNTS. 121 

motors coupled in series, while if alternating currents were 
used, one would have the advantage of employing a single 
machine of equivalent capacity. The principal disadvantage 
of direct current machinery is the commutator, which at high 
voltages is likely to be sooner or later the source of consider- 
able trouble. Careful mechanical and electrical construction 
may materially reduce this difficulty, but it alwaj^s remains to 
be faced, and is liable at any time to become troublesome. 

On long lines, the direct current has the advantage of pro- 
ducing no inductance in the line, an advantage, however, 
which does not apply to plants which can advantageously be 
operated as single units. Such a single unit system, arranged 
for alternating currents, can have the inductance of the cir- 
cuit completely nullified by the simple expedient of strength- 
ening the field of the motor. 

It must be remembered, however, that in several particulars 
continuous current has peculiar advantages. In the first 
place, it is well known that a direct current is decidedly less 
dangerous than an alternating current of the same nominal 
voltage, so far as the question of life is concerned. The differ- 
ence between the two is even greater than would be indicated 
by the difference in maximum voltage. 

An alternating current has a maximum voltage of approxi- 
mately 1.4 times its mean effective voltage, and in addition to 
this an alternating current is certainly intrinsically more 
dangerous by reason of the greater shock to the nervous 
system produced by the alternations of E. M. F. The ease 
of tranforming alternating current to a lower voltage partially 
obviates this objection, but the fact remains. So far as 
danger of fire is concerned, the continuous current has the 
power of maintaining a much more formidable arc than an 
alternating current of the same effective voltage; but, on the 
other hand, the alternating current has somewhat greater 
maximum voltage with which to start the arc, so that, practi- 
cally, honors are even. 

The increase of experience with resonance and kindred 
phenomena, acquired on long lines and at high voltages, has 
emphasized the fact that alternating transmission work is not 
exactly a bed of roses for the engineer, and when it comes to 



122 ELECTRIC TRANSMISSION OF POWER. 

a question of transmission at 60,000 volts or so, difficulties 
multiply. At and above this pressure, there can be little 
doubt that insulation is a very difficult task, and there is 
equally little doubt that of two lines, one constant current 
and the other polyphase, transmitting the same energy at the 
same effective voltage, the former would be in trouble much 
less frequently than the latter. In the first case there are but 
two wires involved, while in the second there are certain to be 
three, and generally considerations of inductance would lead 
to not less than six in a plant of large size. And when the 
point is reached where insulation is a costly matter, the extra 
wires and precautions may easily outweigh any intrinsic saving 
in copper. The constant current plant, too, always has the 
advantage that it is only working at its maximum voltage 
during the peak of the load and the rest of the time has a 
very considerable factor of safety. 

Whether the increased cost and complication of the gener- 
atmg station of a constant current system can ever be endured 
for the sake of these advantages is a matter open to discussion ; 
it certainly cannot be answered in the negative offhand, how- 
ever. The continuity of service possible in an alternating 
plant materially above 60,000 volts is an unknown quantity, 
and in the absence of data upon this point one is not justified 
in estimating the importance of an alternative method. 

Eecent experiments seem to indicate that the insulation 
adequate, say for 50,000 volts a. c, will stand something like 
100,000 volts d. c, without reducing the factor of safety. Part 
of this difference is of course due to the fact that the crest of 
the a, c. wave is 40 to 50 per cent above the nominal rated 
voltage. For the rest, a direct current is free from the surges 
and minor resonance that impose added strains on the a. c. cir- 
cuit. How far this advantage is practically important depends 
mainly on the ability of the insulator maker to produce insula- 
tors capable of use with adequate factors of safety at a. c. volt- 
ages much above those now in use. Some promising insulators 
designed to carry the wire in suspension have recently been in- 
troduced, and are said to test out successfully on pressures as high 
as 250,000 to 300,000 volts. If these claims are substantiated 
the cases in which direct current would be preferable on account 
of easier insulation would be very few. In the matter of one of 



TRANSMISSION BY CONTINUOUS CIRCUITS. 123 

the great dangers to an overhead line and apparatus, i.e., injury 
from lightning, direct current has a very material advantage in 
that it is possible to use coils of considerable self-induction in 
connection with such circuits, so as to keep oscillatory or impul- 
sive discharges out of the machines. This is well shown in the 
singular freedom of arc lighting stations from serious damages 
to the machines by lightning, as compared with stations contain- 
ing other kinds of electrical apparatus. In this case the mag- 
nets of the arc machines themselves act as a powerful induc- 
tance, tending to throw the lightning to earth. High voltage 
shunt-wound dynamos and alternators are much more sensitive 
in this respect. 

Consequently, part of the price one has to pay for the privi- 
lege of utilizing alternating currents is extra care with respect 
to protective devices against lightning. In the present state 
of the art, the best field for combined transmission and dis- 
tribution of power by continuous currents is in cases involving 
distribution over moderate distances, within, say, a couple of 
miles from the centre of distribution, and even then in prob- 
lems where lighting is not an essential part of the work. The 
voltage of a lighting circuit is determined by the voltage of 
the lamps which can be employed upon it, and this is so 
limited that if lighting is to be done on the same circuit as 
power distribution there are few cases where such a com- 
bined system can be successfully used with continuous cur- 
rents. The field seems at present to be somewhat widened 
by the advent of 3-wire systems at 220 to 250 volts on a side, 
but their place in the art is hardly yet secure, although their 
use is extending. 

At all long distances continuous current is at a disadvantage 
in point of available voltage where distribution is to be done, 
and has in most cases no very material advantages for single 
unit work. It will require considerable further advance in 
dynamo building to render continuous current thoroughly 
available for high voltages, and even then only in units of 
moderate size, say 300 to 400 KW. In this lies its weakness. 
Its strength is largely in its present firm foothold in electrical 
practice, and in the fact that standard apparatus for continu- 
ous currents is available everywhere^ and is manufactured 



124 ELECTRIC TRANSMISSION OF POWER. 

cheaply in large quantities by numerous makers. It is, fur- 
thermore, interchangeable to a degree which will never be true 
of alternating-current machinery until there is far greater 
unity in alternating-current practice than we are likely to have 
for some years to come. 



CHAPTER IV. 

SOME PROPERTIES OF ALTERNATING CIRCUITS. 

We have already seen in Chapter I that the current nor- 
mally produced by a dynamo-electric machine is an alternating 
one, so that a continuous current exists in the external circuit 
only in virtue of the commutator. Until within the last 
fifteen years the original alternating current was utilized to 
but a trivial extent. Nevertheless, it possesses certain prop- 
erties so valuable that their practical development has wrought 
a revolution in applied electricity. 

To describe these properties with any degree of complete- 
ness would require several volumes the size of the present, and 
would involve mathematical considerations so abstruse as to be 
absolutely unintelligible to any save the professional reader. 
We shall therefore at the very start drop the academic methods 
of treatment and confine ourselves, so far as possible, to the 
physical facts concerning those properties of alternating cur- 
rents which have a direct bearing on the electrical transmission 
of energy. This discussion will therefore be somewhat uncon- 
ventional in form, although adhering rigidly to the results of 
experiment and mathematical theory. The student who is 
interested in the exact development of this theory will do 
well to consult the excellent treatises of Fleming, Mascart 
and Joubert, Bedell and Crehore, and Steinmetz, all of which 
are full of valuable demonstrations. 

The fundamental differences between the behavior of con- 
tinuous and of alternating currents lie in the fact that in the 
former case we deal mainly with the phenomena of a flow of 
electrical energy already steadily estabhshed, while in the 
latter case the phenomena of starting and stopping this flow 
are of primary importance. These differences are akin to those 
which exist between keeping a railway train in steady motion 
over a uniform track, and bringing it up to speed from a 
state of rest. In steady running the amount of, and the 

125 



126 



ELECTRIC TRANSMISSION OF POWER. 



variations in the power needed, depend almost wholly on the 
friction of the various parts, while in starting both the power 
and its variations are profoundly affected by the inertia of the 
mass, the elasticity of the parts, and other things that cut 
little figure when the train is up to a uniform speed. 

The characteristic properties of alternating currents are due 
mainly to the starting and stopping conditions, and are only in- 
cidentally affected by the circumstance that the flow of current 
alternates in direction. As, however, this alternating type of 
current is in general use and its uniform oscillations give the 
best possible opportunity for observing the effect of repeated 
stops and starts, we will look into the generation of alternat- 
ing current, not forgetting that for certain purposes we shall 




Fig. 43. 



have to recur to the phenomena of a single stop or start in the 
current. 

Fig. 43 shows an idealized generator of alternating currents. 
It is composed of a single loop of wire arranged to turn con- 
tinuously in the space between the poles of a magnet. This 
space is a region of intense electromagnetic stress directed 
from pole to pole, as indicated by the dotted lines. The two 
ends of the loop are connected to two insulated metallic rings 
connected by brushes to the terminals A and B of the external 
circuit. We have already seen that what we call electromotive 
force appears whenever the electromagnetic stress about a 
conductor changes in magnitude. . Now, in turning the loop as 
shown by the arrow, the electromagnetic stress through it 
changes, and of course sets up an electromotive force. In 
the initial position of the loop shown in Fig. 43, it includes 
evidently the maximum area under stress; after it has turned 
through an angle a, this area will be much lesseAed, and when 



PROPERTIES OF ALTERNATING CIRCUITS. 127 

a = 90°, the loop will be parallel to the plane of the electro- 
magnetic stress, and hence can include none of it at all. 

But the resulting electromotive stress is, other things being 
equal, proportional to the rate at which work is expended in 
uniformly turning the coil, i.e., it is proportional to the rate 
of change in the electromagnetic stress included by the coil. 
This rate is, during a single revolution, greatest when the 
sides of the loop are moving directly across the lines of stress, 
and least when moving nearly parallel to them. Hence, we see 
from Fig. 43 that the electromotive force in our coil will be a 
maximum when a = 90° or 270° and a minimum in the two 
intermediate positions. For a simple loop it is easy to compute 
exactly the way in which the electromotive force will vary as 
the loop turns. The area of strain included by the coil in any 




Fig. 44. 

position is proportional to the cosine of the angle a, hence, for 
uniform motion the rate of change in the area is proportional 
to the sine of a. Therefore, the E. M. F. at every point of the 
revolution is proportional to sine a. 

If now we draw a horizontal line and measure along it equal 
distances corresponding to degrees, and then, erecting at each 
degree a line in length proportional to the sine of that par- 
ticular angle, join the ends of these perpendiculars, we shall 
have an exact picture of the way in which the E. M. F. of 
our loop rises and falls. Fig. 44 is such a curve of E. M. F. — 
a so-called ''sine wave," Avhich is expressed by the equation, 
E = E^ sin at. 

This simple form of E. M. F. curve — the ''sine wave" — is 
assumed to exist in most mathematical discussions of alternat- 
ing current to avoid the frightful complications which would 



128 



ELECTRIC TRANSMISSION OF POWER. 



result from assuming such E. M. F. curves as often are found in 
practice. This assumption is somewhat rash, for a true sine 
wave is never given by any practical generator, but the error 
does not often invalidate any of the conclusions, for the exact 
form of the wave only matters in discussing certain cases, where 
it can often be taken into account without much difficulty. 

Actual alternating generators give curves of E. M. F. greatly 
influenced by the existence of an iron armature core which 
collects the lines of force so that as the core turns the change 
of stress through the armature coils is not directly propor- 
tional to anything in particular. A glance at the rudimentary 
dynamo of Fig. 8, Chapter I, will suggest the reason. It is evi- 





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Fig. 45. 



dent enough that the armature could turn almost 30° from the 
horizontal with scarcely anj^ change in the magnetic relations 
of the coils. The result would be a wave with a very flat and 
depressed top, since the rate of change of induction would be 
very moderate when it should be considerable. The practical 
bearings of wave form on power transmission work will be taken 
up in the next chapter. At present it will suffice to say that the 
best standard alternators give a fairly close approximation to 
the sine form. Fig. 45 shows the E. M. F. curve of one of the 
great Niagara generators. This is an excellent example of 
modern practice and show^s a form slightly flatter than the sine 
curve and with a mere trace of depression at the crest. Plenty 
of machines are in operation, however, that give curves not 
within haihng distance of being sinusoidal — e.g. Fig. 46, which 
shows the E. M. F. curve of one of the earliest alternators 



PROPERTIES OF ALTERNATING CIRCUITS. 



129 



designed for electric welding. In this case there is a far sharper 
wave than the sinusoidal, of a curious toothed form. Many of 
the early alternators with ironclad armatures gave curves quite 
far from the sine form, generally rather pointed, while the 
tendency in recent machines has been rather in the opposite 
direction, toward curves like Fig. 45, although seldom so nearly 
sinusoidal. The general equation for their E. M. F. is, 
E = EiSmat + E^sm3at + E^sin5at . . . £'(2«-i)sin (2n— 1) ai. 



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"""""■ 



Fig. 46. 



In other words, the E. M. F. is built up of the fundamental 
frequency and its odd harmonics. 

Now as to the current produced by this oscillating electro- 
motive force. In ordinary work with continuous currents, the 
current corresponding to each successive value of the E. M. F. 
would be very easy to determine by simple reference to Ohm's 

E 
law, C = — ' If the dynamo of Fig. 43 gave one volt maximum 
R 

E. M. F. and were connected through a simple circuit of one 

ohm resistance the maximum current would be one ampere, 

and the current at all points would be directly proportional to 



130 ELECTRIC TRANSMISSION OF POWER. 

the voltage. Hence if the E. M. F. varied as shown in Fig. 44, 
the current would vary in precisely the same manner, and the 
curve showing its variation would, if drawn to the same scale, 
exactly coincide with the curve of Fig. 44. This would be 
generally true if we had only resistance to consider, and the 
treatment of alternating currents would then be very simple. 

But the starting and stopping of current w^hich takes place 
periodically in alternating circuits produces great changes in 
the electro-magnetic stresses about the conductors, and these 
changes are in turn capable of very important reactions. 
They give to the alternating current its most valuable proper- 
ties, but also involve its action in very curious complications. 

Turn back to Chapter I and examine Fig. 4. We see from 
it that whenever the electro-magnetic stresses about a circuit 




Fig. 47. 

as A, change by the variation of the current flowing in it, an 
E. M, F. is set up in the parallel circuit B, opposing the 
change of E. M. F. in A. This fact, as w^e shall see later, is 
the root of the alternating-current transformer. Suppose now 
that in the circuit of our alternator is a coil of wire wound in 
close loops as shown in Fig. 47. Here A and B are the dyna- 
mo terminals, C the general circuit, and D the aforesaid coil. 
Let an E. M. F. be started in the direction A C D. The result- 
ing current flows through D, but the electro-magnetic stresses 
set up by the current about, for instance, the loop e, produce 
an E. M. F. in neighboring coils tending to drive current in 
a direction opposite to that from the dynam.o. In other 
words, e acts toward / just as A acted toward B in Fig. 4. 
Thus each turn tends to oppose the increase of current in 
the others. "When the current in C ceases to vary, of course 
the reactive E.M. F.'s in e and / stop, for there is for the moment 
no change of stress to produce them, but as the main current 
begins to decrease, the reactive E. M. F.'s set in again. 



PROPERTIES OF ALTERNATING CIRCUITS. 131 

The main or '^impressed" E. M. F. is thus opposed in all 
its changes by the reactive or ^^nductive" E. M. F. due to 
the combined action of the loops at D. Hence ttie impressed 
E. M. F. in driving current through the system has to over- 
come, not only the resistance of the conductors, but opposing 
electromotive forces. Therefore since a part of the impressed 
E. M. F. is taken up with neutralizing the inductive E. M. F., 
only the remainder is effective against the true resistance of 
the circuit. Ohm's law, then, cannot apply to alternating cir- 
cuits in which there is inductive action, except in so far as we 
deal with the ''effective" E. M. F. The relation between the 

E E 

impressed E. M. F. and the current is not C = — ; but C = — 

R R 




Fig. 48. 

less a quantity depending on the amount of inductive E. M. F. 
encountered. 

This state of things leads to two very important results: 
First, the current in an inductive circuit is less than the im- 
pressed E. M. F. would indicate. Second, this current reaches 
its maximum later than the impressed E. M. F. For the 
current depends on the effective E. M. F., and for each partic- 
ular value of this the impressed E. M. F. must have had time 
to rise enough to overcome the corresponding value of the 
inductive E. M. F. The current is thus damped in amount 
and caused to lag in ''phase" as shown in Fig. 48. The heavy 
line here shows the variations of the impressed E. M. F., and 
the light line the corresponding variations of current in a 
circuit containing inductive reaction — inductance. The dis- 
tance a h represents the "angle of lag," while 6 c is 180° as 
shown in Fig. 44. Very similar relations are found in prac- 
tice, although the lag is often greater than shown in the cut, 



132 ELECTRIC TRANSMISSION OF POWER. 

particularly when alternating motors are in circuit. Fig. 
49 shows the curves of E. M. F. and current from a very small 
alternating motor at the moment of starting. The angle of 
lag in this case is a trifle over 45°, and the curves are much 
closer to sine waves than is usual. 

The inductive E. M. F., as has already been explained, is due 
to the magnetic changes produced by the variation of the cur- 
rent. Just as in the dynamo of Fig. 43, the actual amount of 
E. M. F. is directly proportional to the rate of change in mag- 
netic stress, which is in turn proportional to the change of cur- 
rent. The inductive E. M. F. is therefore at every point 
proportional to the rate of variation of the current. But the 




Fig. 49. 



current wave is, like the impressed E. M. F. wave, still approxi- 
mately a sine curve, for it has been merely shifted back 
through the angle of lag, and although damped, it has been 
simply changed to a different scale. Being still essentially 
a sine curve, its rate of variation is a cosine curve, or, what 
is the same thing, a sine curve shifted backward a quarter 
period, 90°. Indeed, this is at once evident, for, since the 
current varies most slowly at its maximum, the inductive 
E. M. F. must be a minimum at that point, i.e., it must be 
90° behind the current in phase, while since E. M. F. and cur- 
rent vary symmetrically, in general the forms of the two curves 
will be similar. The effective E. M. F. which is actually 
engaged in driving the current is a wave in phase with the 
current it drives, and of similar shape, i.e., a sine curve. 



PROPERTIES OF ALTERNATING CIRCUITS. 138 

We have, then, in an inductive circuit three E. M. F.'s to be 
considered : 

I. The impressed E. M. F., acting on the circuit. 
II. The inductive E. M. F., opposing I. 

III. The effective E. M. F., the resultant of I and II. 

Plotting the respective curves, they bear to each other the 
relation shown in Fig. 50. Kere a is the impressed E. M. F., 
6 the effective E M. F. (or the current) lagging behind a 
through an angle usually denoted by <^, and c is the induc- 
tive E. M. F. 90° behind h. Now since 6 is the resultant of 
the interaction of a and c, and we know that h and c are 90° 
apart in phase, it is comparatively easy to find the exact relar- 
tion between the three. 

For we can treat electromotive forces acting at known angles 
with each other just as we would treat any other forces work- 





FiG. 50. Fig. 51. 

ing conjointly. If for example we have a force A B, Fig. 51, 
acting simultaneously with a force B C, at right angles to it, 
the magnitudes of the forces being proportional to the lengths 
of the lines, the result is the same as if a single force in magni- 
tude and direction A C were working instead of the two com- 
ponents. This is a familiar general theorem that proves par- 
ticularly useful in the case in hand. 

If we take A B equal to the effective E. M. F., and B C equal 
to the inductive E. M. F., then A C is the impressed E. M. F. 
It at once appears that the angle between A B and A C is <l>, 
the angle of lag. Then from elementary trigonometry it 
appears that 

A B = AC cos <j> 

C B = A C sin cl> = A B tsin<t>, hence 

CB 

tan 4> = 

AB 



134 ELECTRIC TRANSMISSION OF POWER. 

We are therefore in a position to determine the three E. M. 
F/s and the angle <f>, knowdng any two of the four quantities. 
Thus for a given impressed E. M. F., E, such as is found on 
any constant-potential alternating circuit, the effective E. M. 
F. which determines the current is given by E cos <f>. As <^ 
grows less and less through decrease of the inductive E. M. F., 
A C, the impressed E. M. F. necessary for a given current 
also decreases, and finally, when <^ becomes zero, A C = A B. 
In other words, the impressed E. M. F. is then simply that 
needed to overcome the ohmic resistance. 

For any particular current, then, A B is directly propor- 
tional to the resistance of the circuit, while C 5 is directly 
proportional to the ''inductance" of the circuit, that prop- 
erty of the particular circuit which determines the inductive 
E. M. F. Calling this / we may redraw Fig. 51 in a very 
convenient form — Fig. 52. Here we see the relation betv/een 
R and / in determining the impressed E. M. F. necessary to 
drive a certain current through an inductive circuit. The 
magnitude of the E. M. F. evidently is yR^ H- P if the units 
of measurement are chosen correctly, and it is always pro- 
portional to this quantity, which is related to the impressed 
E. M. F. as resistance is to the effective E. M. F. 

Hence y K^ + P has sometimes been called "apparent 
resistance." The more general name, however, is impedance, 
which indicates the perfectly general relation between E. M. F. 
and current. If I be zero, as in a continuous-current circuit, 
then the impedance becomes the simple resistance. We can 
now write out some of the general relations of current and 
E. M. F. in alternating circuits as follows, calling E the im- 
pressed E. M. F. as before: 

E 
C = ^" R' + P 
E ^ C ^ R^ + P 

and with respect to the angle of lag, 
/ ^ 

Vi^2 + P = ~ 



PROPERTIES OF ALTERNATING CIRCUITS. 



135 



/ = i^ tan<^ 
7 



tan 9 = 



R 



Hence, knowing the angle of lag and the resistance of a 
circuit, the inductance can be found at once. The angle 
of lag, depending on the, ratio of I and R, must be the same 
for all circuits in which this ratio is the same. Also in 
any circuit of given inductance, increasing the resistance 
diminishes the angle of lag, while of course also diminishing 
the current for a given value of E. In fact, since / does not 
represent work done, for the inductive E. M. F. represents 





Fig. 53. 



merely a certain amount subtracted from the impressed 
E. M. F. by the reaction of the circuit, any process which for 
a given value of E increases the energy actually spent in the 
circuit is accompanied by a diminution of the angle of lag. 

This freedom of the circuit from any energy losses due to 
/ is a fact of the greatest importance. It is fully borne out 
by experiment, and there is besides good physical reason for 
it. For since current and E. M. F. are the two factors of 
electrical energy, there can be no energy when the product of 
these factors is zero. Note now Fig. 53, developed from Fig. 
50. Hence a is the line of zero E. M. F. and current, h the 
current curve for a single alternation, and c the correspond- 
ing curve of inductive E. M. F. 90° behind the current. 
When 6 is a maximum, c is zero, and vice versa. And since 
c is equally above and below the zero line during each alterna- 
tion of current, the average E. M. F. is zero, and therefore 
the average energy throughout the alternation is zero. The 



136 ELECTRIC TRANSMISSION OF POWER. 

same conditions would evidently continue if instead of an 
alternation we took a complete cycle {i.e., the whole curve 
from the time the current starts in a given direction until 
it starts in the same direction again) or any number of cycles. 
Thus / must be entirely dropped out of consideration in dis- 
cussing the question of work done in an alternating circuit. 
And since E differs from E^, the effective E. M. F., only by 
a fimction of /, the energy value of which is zero, the energy 
in the circuit is exactly measured by E^ and the corresponding 
current which, as we have seen, is in phase with it. But 

E^ = E cos <^. 

Hence, multiplying both members of this equation by C to 
reduce to energy, 

Energy = C E^ =^ C E cos <j>. 

That is, the energy in an alternating circuit is equal, not to 
the impressed E. M. F. multiplied by the current, but to their 
product multiplied by the cosine of the angle of lag. The 
product C E is sometimes called the apparent energy to dis- 
tinguish it from C E^, the actual energy. This apparent 
energy is that obtained by measuring the amperes and the 
impressed volts and taking their product. The real energy 
is that which would be obtained by putting a wattmeter in 
circuit. Hence 

watts 

— = cos ^, 

volt-amperes 

a convenient and common method of measuring the angle of 
lag. If in addition the value of I is wanted, it can be obtained 
at once from the expression for tangent <^ already given. 

We thus see that the energy in an inductive circuit is not 
directly proportional to the voltage as measured, but to the 
effective voltage, which is less by an amount depending on 
the inductance. This difference is sometimes referred to as 
the 'inductive drop" in a circuit. The result is that to drive 
a given current through an inductive circuit the generator 
must give a voltage depending on the impedance of the cir- 
cuit. On the other hand, if an inductive circuit be fed from 



PROPERTIES OF ALTERNATING CIRCUITS. 137 

a given impressed E. M. F. the current required to represent 
a given amount of energy exceeds that required in a non-induc- 
tive circuit, in the ratio of 1 to the cosine of the angle of lag. 
The net result, then, of inductance in an alternating circuit is 
to increase the E. M. F. at the generator required to produce 
a given E. M. F. at the load, and to increase the current re- 
quired to deliver a given amount of energy. 

The E. M. F. and current are here supposed to be meas- 
ured in the ordinary way, by properly designed voltmeters 
and ammeters. In power transmission work, inductance in the 
circuit (line or load or both) means that the dynamo has to 
give voltage enough to overcome the impedance of the system 
and still to deliver the proper number of volts at the motor, 
while the motor will take extra current enough to compensate 
for the lag between the E. M. F. at its terminals and the re- 
sulting current. 

The dynamo thus has to be capable of giving a little extra 
voltage, and the motor nmst be able to stand a little extra cur- 
rent. In other words, both machines must have sufficient mar- 
gin in capacity to take care of this matter of lagging current. 

We have already seen the general relation between resist- 
ance, inductance, and impedance. Let us now look into the 
quantity last mentioned so as to see its numerical relation to 
the others. If a circuit has a certain resistance in ohms and 
a given inductance, what is its impedance, i.e., the ratio 
between the measured voltage and the measured current? 

The real question involved is the value of the inductive E. 
M. F. This, like any other E. M. F., is proportional to the 
rate of variation of the electro-magnetic stress which produces 
it. Its total magnitude depends on the rate of variation of the 
current and the ability of this current to set up stresses which 
can affect neighboring conductors as in Fig. 43. This latter 
property depends on the number of turns, their locality with 
reference to each other, and other similar conditions which 
depend simply on the physical nature of the circuit, and so for 
any given circuit are settled once for all. These properties 
are defined on the basis of their net effect, and the ratio of the 
rate of variation of the current to the inductive E. M. F. pro- 
duced by it in a given circuit is usually known as L, the 



138 ELECTRIC TRANSMISSION OF POWER. 

"coefficient of self-induction" of that circuit. The total 
inductive E. M. F. is then equal to L, multiplied by the actual 
rate of current variation expressed in such units as will fit the 
general system by which E, R and other quantities are con- 
cordantly measured. 

Expressed in this way, the rate of current variation in an 
alternating circuit is 2 ir n, where n is the number of cycles per 
second, and tt has its ordinary meaning of 3.1416. Hence the 
inductance of the circuit is numerically 2 tt n L, the last factor 
being dependent on the nature of the circuit and denoting 
the inductance per unit rate of current variation. The 2 tt n 
factor gives the actual rate of current variation, which may 
change to any amount, while L remains fixed. L therefore 
may at all times in a given circuit be expressed in terms of 

C 




any unit that is conveniently related to other electrical units. 

Such a unit inductance is the henry, which is the inductance 
corresponding to an inductive E. M. F. of 1 volt when the 
inducing current varies at the rate of 1 ampere per second. 

If, therefore, L for any circuit is known in henrys, the 
total inductance I is 6.28 n L. 

We are now ready to apply a numerical value to / in Fig. 
52 and the resulting equations. 

For example, let us suppose that a certain alternating circuit 
has a resistance of 100 ohms and L = 0.1 henry. The im- 
pressed E. M. F. is 1,000 volts. What will be the current and 
its angle of lag? Lay off A B, Fig. 54, 100 units long. Then 
at B, to the same scale erect a perpendicular B C, 2 tt n L in 
height. If we are dealing with an alternating circuit of 60 -^ 
per second, such as is often used for power transmission, 2 
TT n L will be 37.7 units high. Now join A C, and the result- 
ing length on the same scale is the impedance in ohms. But 



PROPERTIES OF ALTERNATING CIRCUITS. 139 



AC = V7^2 + p = V1002 + 37.72 _ 106.9 nearly. And since 
the current equals the impressed E. M. F. divided by the im- 
pedance, the current in this case would be 9.36 amperes instead 
of the 10 amperes due if there had been no inductance. And 

since tan <t> = — , it is here .377, which corresponds to an angle 
R 

of 20° 40'. 

Also, since E = C V^M^, we can readily find the im- 
pressed E. M.. F. required to produce in this circuit any given 
current. For C = 10 amperes, E = 1,069 volts, and so on. 

watts 

We have seen that cos <^ = so that m the 

volt-amperes 

case in hand where cos <^ = .936 the actual energy in the cir- 
cuit is 93.6 per cent of that indicated by the readings of volt- 
meter and ammeter. 

This factor, cos <t>, connecting the apparent and the real 
energy, is known as the '^ power factor" of the circuit. 

As I = R tan <^, and in any given case n is known, L can 
readily be obtained from a measurement of lag in a circuit of 
known resistance. It must be remembered, however, that if 
the inductance is due to a coil having an iron core, the value 
of L will change when the magnetization of the iron changes, 
so that results obtained with a certain current will not hold 
exactly for other currents. The values of L found in practice 
cover a very wide range, from a few thousandths of a henry in 
a small bit of apparatus like an electric bell, to some himdreds 
of henrys in the field magnets of a big dynamo. L in fact is 
nearly as variable as R. 

As a practical example in inductance effects we may consider 
the effect of alternating current in a long straightaway circuit. 
Suppose for example we have a circuit 50,000 ft. long composed 
of No. 4 B. & S. copper wires. The wires are about 1 ft. apart 
and about 20 ft. above the ground. What voltage will be 
required to deliver 10 amperes through this circuit at 130 
cycles per second, and what will be the angle of lag? The 
resistance of this wire is 0.25 ohms per 1,000 ft. L, its co- 
efficient of induction, is .0003 henry per 1,000 ft. The total 
resistance of the circuit is then 25 ohms, and its total indue- 



140 



ELECTRIC TRANSMISSION OF POWER. 



tance, /, = 6.28 x 130 x.03 = 24.5. Plotting as before these 

values, in Fig. 55 we have the impedance equal to V25^ + 24.5^ 

= 35 ohms. Hence E must be 350 volts instead of the 250 

that would suffice in the case of continuous current. Tan 

24.5 
<f> = - — - = .98. The corresponding angle is 44° 25'. The 
25 

ratio of impedance to resistance in this case is 1.4:1. This 

ratio, often called the impedance factor, is a very convenient 

way of treating the matter, and tables giving its value for 

common cases \vill be given later. In case of apparatus being 

connected to the circuit, the computation of its effect is easy. 




If it has resistance R^ and inductance P then the total impe- 
dance of the circuit will be V(i^ + R^y + (/ + Py and so 
on for any number of resistances and inductances, the impe- 
dance being always equal to the square root of the squared 
sum of the resistances plus the squared sum of the inductances. 
Thus an inductance added anywhere in circuit changes the 
total impedance and the angle of lag. 

There are several ways of looking at inductance, according 
as one wishes to deal more particularly with inductive E. M. F., 
the changes in electro-magnetic stress which produce it, or 
the energy changes which accompany it. The first point of 
view is the one here taken, in accordance with the definition 
of the henry just given. Hence the henry may be called unit 
inductance, in which case the quantity / which we have been 
considering measures the inductive E. M. F., and since it is 



PROPERTIES OF ALTERNATING CIRCUITS. 141 

the product of the inductance for unit rate of current change 
multipHed by 2 ir n, li is sometimes referred to as inductance- 
speed, now conventionally termed reactance. 

In alternating-current working inductance may easily be- 
come quite troublesome, through the ''inductive drop" in 
the line and the necessity of sometimes delivering a current 
quite out of proportion to the energy. Thus in alternating- 
current lighting plants during the hours of daylight when the 
actual load is small, the current may be of quite imposing size 
from the lag produced b}^ the inductance of the unloaded 
transformers in circuit. The sort of thing which happens may 
readily be figured out. Suppose we are dealing with a trans- 
former or other inductive apparatus having a resistance of 5 
ohms and L = 1 henry. The impedance at 60 <^ will then be 
V 52 + (6.28 X 60 X ly = V 25 + 376.8^ = 377.8 ohms, sub- 
stantially the same as the inductance alone, and under an 
impressed E. M. F. of 1,000 volts the resulting current would 

377 8 
be 2.65 amperes. But tan <^ = — — - =75.56. Hence <i!> = 89°15' 

5 

and cos <^ = .013. Therefore while the apparent energy is 
2.65 X 1,000 = 2,650 watts, the real energy is only 2,650 X.013 
watts = 34 + : really the loss due to heating the conductor. 
This is of course a very exaggerated case, as it takes no account 
of the energy that would be required to reverse the magneti- 
zation in whatever iron core the apparatus might have. It 
does, however, show very clearly that the current flowing 
depends practically on the inductance and very little on the 
resistance, and that the angle of lag is so great that the dis- 
crepancy between apparent and real energy may also be very 
great. In practice cos <^ may fall as low as 0.1 on single 
pieces of apparatus, and ranges up under varying conditions 
of load to .95 or more. 

These practical considerations naturally raise a question as 
to the effect of impedances in parallel. The joint impedance 
of two impedances in series must first be discussed. 

The resistance of two resistances in parallel is of course 
familiar, li R = 2 ohms and R^ = 4: ohms, then their joint 
resistance is the reciprocal of the sum of their reciprocals, 



142 



ELECTRIC TRANSMISSION OF POWER. 



thus, 



(R + R') = ^ = 1 J ohms. 

Y + 4 



We have seen, however, that impedances cannot be added 
in the ordinary manner. If we take two impedances made up 
respectively of i^ = 4, / = 3, and R^ = 6, P = 3, we must pro- 
ceed as in Fig. 56. The first impedance is 5, the second 6.70. 
The true impedance of the two in series is given by the dotted 
hues, and is 11.66, not 11.70. That is, the impedances must be 
added geometrically, since unless <^ = <^j the arithmetical sum 
of the impedances does not represent the facts in the case. 




R-4 



Fig. 56. 



Similarly, while it is perfectly true that the joint impedance 
of two impedances in parallel is equal to the reciprocal of the 
sum of their reciprocals, the summation must be done as in 
Fig. 56 to take account of the difference of phase which may 
exist in the two branches. Taking the data just given, the 
reciprocals of the two impedances are .20 and .149 respectively. 
Drawing these on any convenient scale as in Fig. 57, preserv- 
ing between them the angle due to the difference of phase as 
given by <^ and <^i, we find the geometrical sum of the recipro- 
cals to be .348, of which the reciprocal is 2.87. This is the 
joint impedance of the two which we have thus geometrically 
added. 

This same process can be extended to any number of impe- 
dances in parallel. In a precisely similar way any number of 
directed quantities may be laid off and geometrically added, 
the final sum being in direction and magnitude the line from 



PROPERTIES OF ALTERNATING CIRCUITS. 143 

the starting point to the finish. It is important to note that 
since the currents in such cases are generally not in phase 
with each other, it usually happens that the sum of the currents 
in the branches differs from the current in the main circuit, 
as they are ordinarily measured. It is in fact a prominent 
characteristic of alternating circuits that both currents and 
voltages are liable to vary in a way at first sight very erratic. 
Particularly is this the case when there is capacity in the cir- 
cuit, a condition which we will now investigate. 

By a circuit having capacity, we mean one so constituted 
that E. M. F. applied to it stores up energy in the form of 
electrostatic stress, which starts this energy back in the form 
of current when the constraining E. M. F. is removed. 

Such a condition exists whenever two conductors are 
separated by an insulating medium, or dielectric, as in the 
ordinary condenser of Fig. 58. Here A and B are two metal 

.30 



.348 
Fig. 57. 

plates separated by a layer, C, of some insulating material. 
If now these plates are connected to the terminals of a dyna- 
mo they become electrostatically charged. The electrostatic 
stress tends to draw the plates together, and in addition sets 
up intense strains in the dielectric C, rendering potential 
thereby a certain amount of energy which flows into the 
apparatus in the form of electric current. This energy is 
returned as current if the original electromotive stress is 
removed and A. and B are connected together. The medium 
behaves just as if it were a strained spring, and when it returns 
its energy to the circuit it does so spring-fashion with rapid 
oscillations, dying out the more slowly the less resistance they 
encounter. 

The capacity of such a condenser is the quantity of energy 
which it can store up as electrostatic strains in C. It is pro- 
portional to the area of the plates, to the E. M. F. produc- 
ing the strains, and to the '^ dielectric constant" of C, that is, 
the coefficient which for that particular substance measures its 



144 ELECTRIC TRANSMISSION OF POWER. 

power to take up electrostatic strains. Oddl}^ enough the 
capacity decreases as C grows thicker, indicating that the 
intensity of the strain is the thing which counts rather than 
the vohune of dielectric. Without knowing the exact character 
of electrostatic strain, it is difficult to get a clear mechanical 
idea of the state of things which causes the energy stored to 
increase as the thickness of C diminishes. A similar condition, 
however, holds for a wire held tightly at one end and twisted 
at the other; the shorter the wire, the more energy stored for 
a given angle of twist. 

As in the case of inductance, for practical purposes the unit 
of capacity is taken in terms of unit pressure, i.e., one volt. 
Unit capacity, then, in terms of energy, is the capacity of con- 
denser in which one watt-second can be stored under an elec- 



FlG. 58. 

tromotive stress of one volt. This capacity is one farad, and 
as it is many thousand times larger than anything found in 

practice, ^ of it (the microfarad) is more often used. 

When a condenser is used with an alternating current, the 
rate at which energy is stored and delivered evidently increases 
with the frequency, or, what is the same thing, for a given 
alternating E. M. F. the greater the frequency the greater the 
current received and dehvered by the condenser. 

Numerically the current in a condenser of capacity k farads, 
supplied by an E. M. F. of e volts at n cycles per second, is 

C = 2 TT n ek , 

which is simply the current due to e volts and k farads mul- 
tipUed by the frequency expressed in angular measure. Thus, 
if we have a 2 microfarad condenser fed by an alternating 



PROPERTIES OF ALTERNATING CIRCUITS. 145 

E. M. F. of 2,000 volts and 130 cycles per second, the current 

flowing is 

^ 2,000 X 6.28 X 130 X 2 

C = , ^ = 3.26 amperes. 

1,000,000 ^ 

In such an alternating circuit, then, there will be a substantial 

current flowing in spite of the fact that there is a break in the 

conductor at the condenser. In short, the circuit acts as if it 

2,000 

had a resistance of =613 ohms, which is the impedance of 

3.26 ^ 

the circuit. More exactly the innpedance is • It should 

2 TT n /c 

be noted here that some writers refer to this fundamental 
condenser function (2 tt n) A; as capacity-speed. Capacity-im- 
pedance really is a negative reactance, often termed condensance. 

To see the relation which this capacity-impedance bears to 
other impedances in the circuit, it is necessary to look into 
the properties of the E. M. F. of the condenser. As energy is 
stored in the condenser the opposing stresses in it increase 
until the applied E. M. F. can no longer force current into it 
and the condenser is fully charged. At the moment, then,, 
when current ceases to flow, the E. M. F. of the condenser 
tending to discharge it is at a maximum. Hence, since the 
one has a maximum as the other is zero, the E. M. F. of the 
condenser and the charging current are 90° apart in phase. 

But the inductive E. M. F. is also 90° from the current, and, 
as we have seen, lagging. It has its maximum when the 
current is varying most rapidly; and when the strejigth of cur- 
rent in a given direction is increasing, the inductive E. M. F. 
in the same direction is diminishing, as shown in Fig. 53. As 
regards capacity, however, the moment of maximum condenser 
E. M. F. in a given direction is . that at which the current 
thereby becomes zero, so that as the current changes sign it 
has behind it the thrust of the full E. M. F. of the discharging 
condenser, while at the same moment, as we have just seen, 
the opposing inductive E. M. F. is at its maximum. Hence the 
E. M. F. of the condenser has a maximum in one direction 
when the inductive E. M. F. has its maximum in the other 
direction. The two. are thus 180° apart in phase, and each 



146 



ELECTRIC TRANSMISSION OF POWER. 



being 90° from the current, the condenser E. M. F. must be 
regarded as 90° ahead of the current, just as the inductive 
E. M. F. is 90° behind it. 

The condition of affairs is shown in Fig. 59. Here a a is 
the hne of zero current and E. M. F. All quantities above 
this line may be regarded as -\- , and all below it as — ; 6 is a + 

b 




Fio. 59. 



wave of current to which appertains c c the curve of inductive 
E. M. F. lagging 90° behind the current, and d d the condenser 
E. M. F., leading the current 90°. 

It is evident that these two E. M. F.'s always are opposing 
each other — when one is retarding the current the other is 
accelerating it, and vice versa. 

The condenser E. M. F. has no effect on the total energy 




Fig. 60. 



of the circuit for the same reason that held good in respect 
to Fig. 53; it is obviously akin to a spring, alternately receiv- 
ing and giving up energy, but absorbing next to none. 

Capacity may be considered as negative inductance in many 
of its properties. If, as in Fig. 59, it is in amount exactly 



PROPERTIES OF ALTERNATING CIRCUITS. 



147 



equivalent to the inductance, the total effect on the circuit is 
as if neither capacity nor inductance were in the circuit. In 
such case it is as if C B, Fig. 51, should be reduced to zero. 
The impressed E. M. F. then becomes equal to the effective 
E. M. F., the angle of lag vanishes and the circuit behaves as 
if it contained resistance only. If the condenser E. M. F. is 
not quite large enough to annul the inductance it simply 
reduces it. 

Fig. 60 illustrates the effect of varying amounts of capa- 
city. In the main triangle ABC, the sides have the same 
signification as in Fig. 51. Since the capacity E. M. F. is 




180° from, i.e., directly opposite to, the inductive E. M. F., the 
effect of adding the capacity E. M. F. C D, is to reduce the 
effective inductance to B D and give as an impressed E. M. F. 
A D and an angle of lag <l>^. Now increasing C D to equal C B, 
the inductance is annulled, <i> becomes zero, and the impressed 
and effective E. M. F.'s are the same. Then increase C D still 
further so that it becomes C E. Now the inductance C B 
not only is neutralized but is replaced by a negative inductance 
B E. The angle of lag now becomes an angle of lead, <f>2, the 
necessary impressed E. M. F. rises to A E, and the circuit 
behaves as regards the relations between current, E. M. F., and 
energy, just as it did when affected by inductance. There is 
the same discrepancy between real and apparent energy, the 
same necessity for more current to represent the same energy. 
But adding inductance now decreases the angle of lead. From 
a practical standpoint capacity by itself is objectionable, but 
capacity in a line containing inductance is sometimes a very 
material advantage. 



148 ELECTRIC TRANSMISSION OF POWER. 

The nature and reality of this curious phenomenon of 
'heading" current in an alternating circuit may be appre- 
ciated by an examination of Fig. 61. This shows the actual 
curves of current and E. M. F. taken from a dynamo working 
on a condenser in parallel with inductance. The maximum of 
the current wave is very obviously in advance of the maximum 
of the E. M. F. wave, though by a rather small amount (ac- 
tually about 6°) . The capacity in this case was between 2 and 
3 microfarads. 

Treating capacity as a negative inductance enables us to 
compute its effects quite easily. We have already seen how 
to reckon the impedance of a condenser; using the word im- 
pedance here in its proper sense of apparent resistance by 
whatever caused. This quantity we can add geometrically to 
the ohmic resistance of a circuit and obtain the net impedance 
just as in Fig. 54. We must bear in mind, however, that the 
capacity E. M. F. is 180° from the inductance E. M. F., though 
each is at right angles to the effective E. M. F. which is con- 
cerned with the ohmic resistance. 

Instead, then, of computing the total impedance as 

Vr^ + P^ it becomes y r^ + f^; jJ ' ^^^ second term under 

the radical being the square of the apparent resistance due to 
the capacity, just as P expressed the square of the apparent 
resistance due to inductance. 

Suppose, for example, we have a resistance of 100 ohms in 
series with a condenser of 4 microfarads capacity. The 
impressed E. M. F. is 2,000 volts at 130 cycles per second. 
What is the total impedance, the resulting current, and the 
angle <^, in this case an angle of lead? Here 

6.28 X 130 X 4 ^_^ 

2 Trn /c = = .003266 

1,000,000 

^ = 306. 



2'ir nh 



Laying off the resistance A B in Fig. 62 as in Fig. 54, and 

drawing to the same scale at right angles (downward to 

2 tr nk 



PROPERTIES OF ALTERNATING CIRCUITS. 



149 



emphasize its opposition to the inductance of Fig. 54), we 
have for the length of the diagonal A C, which represents the 
total impedance, VlOO^ + 306^ = 322 ohms. The current 
flowing is then, 6.21 amperes. The angle <^ is determined as 

before hy tsm <f> = = 3.06, whence <^ = 72°, cos <^ = .309, 

so that we are dealing with a ^' power factor" like that pro- 
duced by a heavy inductance, although the current leads the 
E. M. F. instead of lagging behind it. If we consider an 
inductance in series with this circuit, we should have to reckon 




Fig. 62. 



it upward in Fig. 62, thereby subtracting it from the former 
length B C. 

Suppose for example for the given inductance L = .3 henry. 
Then I = 2 tt n L = 245. If in Fig. 62 we draw 245 on the 
scale already taken, upward from C, we shall reach the point 
D. B D therefore is 61, and A D, the resulting impedance, 
117 ohms. 



is V1002 + 61 
2,000 



fore 



The new current is there- 
17.09, and as tan <l>^ = .61, </>i = 31°.5, being still 



117 
an angle of lead. 

It is easy to see that for a certain value of /, the capacity 
effect and inductance effect would exactly balance each other. 



150 ELECTRIC TRANSMISSION OF POWER. 

This value is obviously 2 Trn L = , since then in Fig. 

2 IT n k 

62, B C — C D = 0, and the impedance and resistance are 
the same thing, while <^ becomes zero. 

In actual circuits the capacity is seldom in series with the 
inductance. It is usually made up of the aggregated capacity 
of the line wires with air as the dielectric, the capacity of 
any underground cables that may be in circuit, and finally 
the capacity of the apparatus, transformers, motors, and the 
like, that may be in circuit. Generally the major part of the 
total inductance is in the apparatus rather than the line, and 
hence in parallel with the capacity. In many cases nearly 
all the inductance and capacity is due to the apparatus, and 
the two may be regarded as in parallel substantially at the 



ul 


A Bg 


01 


C3 



Fig. 63. 



ends of the line. The inductance of generators and trans- 
formers may amount to several henrys, while their capacity is 
by no means small, though very variable, like the inductance. 
For example, the capacity of a large high-voltage generator or 
transformer may often amount to several tenths of a micro- 
farad. Armored or sheathed cable has a capacity of from a 
quarter to a half or more, microfarad per mile. Altogether 
one may expect to find a capacity of several microfarads 
frequently, and large fractions of a microfarad very often. 

Suppose now we have in parallel a capacity A , Fig. 63, of 2 
microfarads, and an inductance of .5 henrys, the resistance con- 
nected with each being insignificant. Assuming as before 2,000 
volts and 130 cycles, what is the total impedance of the com- 
bination, and the resulting current? We have already seen 
how impedances in parallel are to be treated. In the case in 

hand the impedance of A is =613 ohms, 

^ 6.28 X 130 X .000,002 

and that of B is 6.28 X 130 X .5 = 408 ohms. Now remem- 



PROPERTIES OF ALTERNATING CIRCUITS. 



151 



bering that in adding impedances their geometrical sum is to 
be taken, and that joint impedance is the reciprocal of the 
geometrical sum of the reciprocals of its components, we can 
proceed as follows : The reciprocal of 613 is .00163. This we 
will lay off to any convenient scale just as in Fig. 57. As it is 
capacity-impedance, we will draw it downward for the sake of 
uniformity, making A B, Fig. 64. Now take the inductance. 
The reciprocal of 408 is .00245. As the inductance and capa- 
city E. M. F.'s are here as before at an angle of 180°, we must 
draw this upward from B, giving us the distance B C. The 
geometrical sum is then A C = .00082, of which the recip- 



B 
Fig. 64. 



rocal gives the resultant impedance as 1,219 ohms. Hence 



the net current in the line under 2,000 volts is 



2,000 
2,219 



= 1.6 



ampere. But under the same pressure the current in A would 
obviously be 3.26 amperes and that in the inductance B would 
be 4.90 amperes. We have then the curious phenomenon of 
a total current in the line smaller than that through either of 
the two impedances in circuit. It is as if A and B formed a 
local circuit by themselves in which the condenser A served 
as a species of generator. It is quite evident that the total 
energy of the system, however, is that due to the current in 
the line, so that the phases in A and B are greatly displaced. 
If the resistances in the circuit were quite negligible, the net 
current in the line would be indefinitely small when A = B, 



152 ELECTRIC TRANSMISSION OF POWER. 

that is, when L = - • Of course, however, the true impedances 
k 

of both A and B are modified by the resistances, however small, 

so that in Fig. 64 the impedances will always be at a small angle 

with the Fj. M. F.'s instead of being coincident. Hence the net 

current can never become zero, though when the impedances 

of A and B are large compared with the resistances, the line 

current will be very small when L = - • 

This case is in sharp contrast to that in which condenser 
and inductance are in series with each other. For then the 

line current is increased as L approaches - instead of becom- 

k 

ing smaller relatively to the branch currents, although in each 

case the same relation between capacity and inductance gives 

the maximum ''power factor" on the circuit, since whatever 

the current, under this condition it depends most nearly on the 

resistance alone. When the resistance is quite perceptible in 

comparison with the impedances of A and B, we should form a 

resultant impedance with each, and then combine the two 

somewhat as in Fig. 56. 

If then we have an inductive load of any kind in circuit, a 
condenser in parallel therewith will reduce the current on the 
line and thereby increase the ''power factor" of the system. 
It does this, too, without any material loss of energy and with- 
out necessarily increasing the amount of current flowing through 
the inductance under a given E. M. F. on the line. Were 
condenser and inductance in series, the power factor could 
likewise be improved up to a certain point, but trouble would 
be encountered in that the condenser would necessarily have 
to be large enough to let pass enough current to supply the 
energy required in the inductance at full load. 

In all practical cases the relations between resistance, 
capacity, and inductance, which have just been set forth, are 
somewhat modified by the existence of losses of energy in the 
circuit quite apart from these due merely to overcoming of 
resistance. Energy is required to reverse the magnetization 
of the iron cores of inductance coils, and to reverse the electric 



PROPERTIES OF ALTERNATING CIRCUITS. 



158 



strains in the dielectric of condensers. It therefore happens 
that with a condenser in circuit its apparent current is not 
exactly 90° ahead of its impressed E. M. F., as shown in Fig. 
59, but a trifle less, so that the current has a small component 
in phase with its E. M. F., thus supplying the energy in ques- 
tion. The deviation from 90° is generally but a small fraction 
of a degree. The same sort of thing happens when an induc- 
tance having an iron core is in circuit. However small the 
resistance, the lag still misses 90° by enoug-h to take account 
of the energy required for magnetic losses. The variation 
from 90° in this case may amount to 30° or more. Hence 



RESISTANCE 




Fig. 65. 




the failure to take account of these energy losses in the exam- 
ple given on page 141. 

The result is that in adding inductance and capacity effects, 
one sometimes seems not to get so simple results as in Fig. 64, 
but something more like Fig. 65. Here it is clear that no com- 
bination of capacity and inductance can leave the circuit free 
from everything except resistance, for both the inductance and 
the capacity demand energy in the circuit bej^ond that ex- 
pended in the resistance. Evidently, however, <^ may be 
reduced to zero if the relation between capacity and induc- 
tance is just right. Thus while the lag may be reduced to 
zero, no combination can dodge the energy losses. Whenever 
all the energy losses are taken into account, the true induc- 
tance and capacity E. M. F.'s will be found 180° apart and 
90° from, the energy, exactly where they belong. 

Closely connected with this subject is the matter of reso- 



154 



ELECTRIC TRANSMISSION OF POWER. 



nance, which will be taken up in connection with the discus- 
sion of the line. Briefly the phenomenon is this: We have 
seen that the E. M. F. of a condenser is a maximum, when the 
current is zero, so that as the current changes sign the thrust 
of the condenser E. M. F. is behind it. Now if the condenser 
E. M. F. synchronizes with this current, the impressed E. M. F. 
is added to it, imposes an added stress on the condenser dur- 
ing the next alternation, catches therefrom an additional kick 
as it passes through zero again, and so on. Thus the net 
effective E. M. F. is raised by the action of the condenser, 
and would increase enormously but for its being frittered 
away in overcoming resistance and supplying such energy 




Fig. 66. 



losses as we have just been considering. By avoiding these 
losses as far as possible, one can actually raise the voltage on 
an alternating circuit to twenty-five or thirty times its nominal 
amount by employing a condenser of the proper capacity. 
Even when the impressed E. M. F. and the current are not 
quite in phase, one has always a component of the condenser 
E. M. F. tending to act in a similar manner. Whether it 
actually produces a sensible rise of voltage depends on its rela- 
tions to the frequency and resistance with which it has to deal. 
In fact, it is the addition of this same condenser E. M. F. to 
the circuit that enables one to neutralize inductive E. M. F. 
Whether or not the neutralization of inductance by capacity 
produces a real resonant rise of voltage depends on the fre- 



PROPERTIES OF ALTERNATING CIRCUITS. 155 

quency and whether the energy losses are small or large. If 
they are small enough to let the sum of the impressed and the 
condenser E. M. F. accumulate during several alternations 
there will be a noticeable increase of voltage, otherwise not. 

The dynamics of resonance may perhaps be best understood 
by a very pretty mechanical analogue due to Dr. Pupin. The 
apparatus on which it is based is shown in Fig. 66. It is a 
torsional pendulum composed of a heavy bar A suspended by 
a stiff elastic wire B, from a light circular bearing plate C. 
This plate rests in a recess a, with a frictional resistance which 
can be regulated by the screw shown in the cut. Such an 
apparatus acts much like an electric circuit, having induc- 
tance, capacity, and ohmic resistance. The moment of inertia 
of the bar A corresponds to self-induction, the elasticity of 
B to condenser capacity as we have just noted in connection 
with Fig. 58, and the friction of C to the resistance. More- 
over, if / is the moment of inertia of the bar A, and B the 
reciprocal of the elastic capacity of the wire, then within cer- 
tain values of the frictional resistance the oscillation period of 
the pendulum thus formed is, in seconds, 



r = 27r ^flB. 

This corresponds most beautifully to the time constant of an 
electric circuit, which is, if the energy losses are within cer- 
tain limits, 

2 



T = -^^ Vl C, 
1,000 

wherein L is in henrys, C the capacity in microfarads, and 
the denominator comes from the units being thus chosen. 

Now, if this pendulum be given a twist it will oscillate at 
constant frequency until the friction gradually brings it to 
rest with oscillations of steadily decreasing amplitude. If, 
however, at the end of each complete swing it should receive a 
slight push, its oscillations would continue and would increase 
in amplitude up to a limit set by the frictional resistance. 
The condition for such permanent increase of amplitude is 
that the frequency of the pushes must coincide with the period 
of the pendulum. In the electrical case, resonance thus 



156 ELECTRIC TRANSMISSION OF POWER. 

occurs when the frequency and the time constant of the cir- 
cuit are equal. Further, maintaining our auxiliary pushes at 
their original frequency, suppose I to be decreased by taking 
weight off A progressively. As the time constant of the 
pendulum thus diminished a point would be found, and that 
very soon, at which resonance would cease, and the same 
result would follow increase of 7, so that when the circuit 
begins to get out of tune the resonance soon becomes rather 
trivial. If, however, the pushes were supplemented by others 
of 3, 5, 7, etc., times the frequency, corresponding to the 
harmonics found in an ordinary alternating circuit, new points 
of resonance would appear when the period of A assumed 
corresponding values. 

As to the magnitude of the resonant effect, in the torsional 
pendulum case the amplitude evidently increases with the 
strength of the pushes, their absolute frequency, which mea- 
sures the energy supplied, and the moment of inertia of A, 
which stores this energy. It decreases in virtue of the fric- 
tional resistance. Corresponding reasoning holds in the elec- 
trical case, and to a first approximation the E. M. F. in a 
completely resonant circuit is 

n L 

in which E is the impressed E. M. F. concerned, L the induc- 
tance in henrys, R the resistance in ohms, and n the frequency. 
In case of resonance with harmonics, n and E refer to the 
frequency and magnitude of the harmonic implicated, and £" 
becomes a resonant component of the E. M. F. wave. This 
subject will be discussed more at length in Chapter XIII. 

We have now glanced at the most striking characteristics 
of alternating currents — those concerned with the phenomena 
of inductance and capacity. 

It remains to note very briefly some other physical proper- 
ties that are of practical importance. 

The most important single property of alternating current 
is the ease with which it can be changed inductively from one 
voltage to another. If a circuit carrying such a current is put 
in inductive relation with another circuit as in Fig. 4, Chapter 



PROPERTIES OF ALTERNATING CIRCUITS. 157 

I, the electro-magnetic stresses set up by the first circuit can be 
utilized to produce alternating current of any desired voltage 
in the second circuit. The details of the operation will be 
taken up later; suffice it to say here that it is essentially the 
transformation of the electro-magnetic energy due to one 
circuit into electrical energy in another circuit. 

Alternating currents can be regulated in amoimt by putting 
inductance in the circuit without losing more than a very 
trifling amount of energ}^ This very property, however, is 
troublesome when an alternating current is used for magnetiz- 
ing purposes. It is very difficult to get a large current to flow 
around a magnet core because of the high inductance, and even 
then the magnetic and other losses in the core are serious unless 
great care is taken. These difficulties have stood in the way 
of getting a good alternating motor until within the past few 
years, and even now such motors have to be designed and con- 
structed with the greatest care to avoid trouble from induc- 
tance and iron losses. For some classes of work, such as teleg- 
raphy and electrolytic operations, the alternating current is 
ill suited save under special conditions and with special appar- 
atus. For the general purposes of electrical power transmis- 
sion it is singularly well fitted, from the great ease with which 
transformations of voltage can be made, certain very valuable 
properties of the modern alternating motor, and the great 
simplicity and efficiency with which regulation can be effected. 
The only inconvenience attending transmission by alter- 
nating current is that incurred when direct current must for 
one reason, or another, be supplied. This is in a fair way to 
be greatly reduced by both increasing use of alternating cur- 
rent in distribution, and by improvement in apparatus for 
obtaining direct current from an alternating source. 



CHAPTER V. 

POWER TRANSMISSION BY ALTERNATING CURRENTS. 

Broadly considered, we may say that all systems of trans- 
mitting power by alternating currents are closely akin in 
principles and characteristics. The growth of the art, how- 
ever, has proceeded along several lines, and certain conven- 
tional distinctions have come to be observed in considering 
the methods employed for rendering the alternating current 
applicable to the working conditions of power transmission. 

Alternating systems are usually classified as either mono- 
phase or polyphase. By the former term is generally imder- 
stood a system generating, transmitting, and utilizing a simple 
alternating current such as shown in diagram in Fig. 44. By 
the latter is meant a system generating, transmitting, and 
utilizing two or more such currents differing in phase and 
combined in various ways. As regards the systems, this dis- 
tinction is sufficiently sharp, but as regards individual parts of 
such systems the line of demarcation is sometimes hazy, since 
a monophase current may be the source of derived polyphase 
currents, and on the other hand polyphase currents may be so 
combined as to give a monophase resultant. Mixed systems 
involving unsymmetrical phase relations may properly be 
called heterophase. 

As regards apparatus, any device that performs all its func- 
tions in a normal manner when deriving all its energy from a 
simple alternating current should be classified as monophase. 
If its functions require the cooperation of energy received 
from two or more alternating currents differing in phase, 
the apparatus is essentially polyphase. 

For certain purposes the one system is best adapted, for 
certain other purposes the other is most advantageous, but 
the underlying principles are the same, and the apparatus has 
much the same general properties. 

The material of alternating transmission work may be classi- 

158 



TRANSMISSION BY ALTERNATING CURRENTS. 159 

fied as follows, the transmission line itself being reserved for 
discussion in a separate chapter in connection with other line 
work: 

I. Generators. III. Synchronous Motors. 

II. Transformers. IV. Induction Motors. 

In addition to these, there have been recently introduced 
alternating series-wound motors with commutators which will 
be discussed in their proper place. 

After a tolerably careful examination of the practical prop- 
erties of this apparatus in its various forms,we shall be able 
to appreciate its application to the electrical transmission of 
power under various circumstances. Subsidiary apparatus of 
all kinds will be referred to elsewhere, and the divers systems 
that have been exploited can best be considered after we 
have looked into the characteristics of their component 
parts. 

Alternating power transmission is now going through the 
stage of development that is inseparable from the rise of a 
comparatively new art — the planting time of '^systems," if 
one may be allowed the simile. It is sufficiently certain 
already that the same sort of plant will not do equally well 
under all circumstances. 

The principles of the alternating current dynamo have 
already been explained, but the constructional features of such 
machines are sufficiently distinct from those of continuous 
current dynamos to warrant examination in considerable 
detail. 

The modifications peculiar to alternators are in general due 
to two causes; first, the general use of a fairly high frequency, 
and, second, the necessities of rather high voltage. 

We have already seen that, while an ordinary continuous 
current dynamo fitted with collecting rings will give alternat- 
ing current, the frequency is rather low. To secure a higher 
frequency it becomes necessary to increase the number of poles, 
the speed, or both. Increasing the number of the poles is 
the usual method employed, since continuous current dynamos 
are generally for the sake of keeping up the output operated at 
speeds as high as the conditions of economical use render 
desirable. So we usually find that for equal outputs alternators 



160 



ELECTRIC TRANSMISSION OF POWER. 



have many more poles, 
speed, and frequency is, 



The general relation between poles, 



n = 



60 



where p is the number of poles, N the revolutions per min- 
ute, and n the complete cycles per second. 

For example, belt-driven continuous current dynamos of 100 
to 500 kilowatts usually run at speeds from 600 down to 300, 
and have four or six poles, thus giving 15 to 20 cycles per sec- 




FlG. 67. 



ond, while modern alternators of similar size and speed have 
from 12 to 24 poles, thus adapting them for a frequency of 
30^^ to 60^^. Machines for the older frequencies of 120^ to 
140^ were usually even more liberally provided with poles 
unless driven at speeds considerably above those mentioned. 
The general appearance and design of a typical belted alter- 
nator is shown in outline in Fig. 67. This is a 150 KW gener- 
ator runniag at 600 revolutions per minute, and shows admir- 
ably the general characteristics of rather numerous poles, low 



TRANSMISSION BY ALTERNATING CURRENTS. 161 

base, and massive bearings that nowadays belong in common 
to machines by nearly all makers. Such alternators usually 
have very powerful field magnets, and the projecting pole- 
pieces are usually built up of iron plates like the armature, for 
the same purpose of preventing eddy currents in the iron. The 
ring of field magnets is split on the level of the centre of the 
shaft, for convenience in removing the armature. The weight 




Fig. 68. 



of belt-driven generators of the output named is usually six 
or seven tons. 

This same general type is commonly adhered to whatever 
the nature or voltage of the armature winding, save in the 
case of special machines. 

The winding of a modern alternator is nearly always widely 
different from continuous-current windings. In alternators the 
voltage is generally from 1,000 volts up, seldom below 500 volts, 
and to obtain this the windings corresponding to the numerous 
poles are almost universally connected in series instead of in 
parallel. 

This necessitates connecting the numerous armature coils 
in a very characteristic way. For when a given armature 



162 ELECTRIC TRANSMISSION OF POWER. 

coil is approaching one of the north poles of the field magnet 
and is generating current in a given direction, the next arma- 
ture coil is necessarily approaching the neighboring south pole, 
and if wound in the same direction as the first coil would 
generate a current flowing in the opposite direction. Hence 
if all the armature coils are to be in series, they must be 
wound alternately in opposite directions, as shown in Fig. 68. 
This arrangement throws in series the E. M. F.'s generated 
by all the armature coils. Sometimes for convenience the 
halves of the armature are connected in parallel, thus giving 
half the voltage and twice the current by a simple change in 
connections. Fig. 69 shows in diagram such a winding for a 




FlQ. 69. 

16-pole field, and its relation to the collecting rings. Note 
that each half of the winding preserves the characteristics 
shown in Fig. 68. 

In practical machines as built to-day, the armature coils are 
nearly always bedded in slots in the armature core. The 
early American machines were generally built with smooth 
armature cores, and upon these flat coils were laid and held in 
place by an elaborate system of binding wires. This construc- 
tion has been virtually abandoned by all the principal manu- 
facturers in favor of the so-called ^'iron-clad" armature, which 
has the double advantage of great mechanical solidity and of 
permitting the armature coils to be wound in forms thoroughly 
insulated, and then dropped into place in their slots and 



TRANSMISSION BY ALTERNATING CURRENTS. 163 

firmly wedged in position. The winding is, therefore, very 
little liable to damage and easily replaced if necessary. 

The slotted armature cores are variously arranged in dif- 
ferent macnhies, but always with the same object in view. 

Fig. 70 shows one widely used arrangement of slots. Here 
the coils are wound in forms and thoroughly insulated. They 
are then pushed into place in the previously insulated slot, 
each coil enclosing a single armature tooth. When firmly in 




Fig. 70. 



place the insulating material is put into position above them 
and a hard- wood wedge is driven into the dove-tailed upper 
portion of the slot, holding the coils and their surrounding 
insulation permanently in place. The coils here shown con- 
sist of only four turns of heavy wire. Often there are many 
more turns per coil, and frequently the round wire is replaced 
by rectangular bars. In generators for use with raising trans- 




FiG. 71. 



formers each coil sometimes consists of a single turn of bar 
copper, but whatever the nature of the coil the slots are 
arranged much as here shown. 

Another familiar form of slotted armature is shown in 
Fig. 71. The coils are, as in the case just mentioned, wound 
in forms and solidly insulated. They are then sprimg over 
the armature teeth into place and tightly wedged. The 
slots are carefully insulated also, and by the time the winding 
is completely assembled it is so thoroughly insulated that 
repairs are few and far between. The special peculiarity of 



164 ELECTRIC TRANSMISSION OF POWER. 

this form of core is that the outer corners of the teeth are cut 
away, so that the coils come more gradually into the field of 
the pole-pieces than if the edges were sharp. The object of 
this device is to obtain a curve of E. M. F. more nearly accord- 
ing with the sine wave form, and experience show^s that the 
plan works successfully. Without such precautions the E. M. F. 
curve is very likely to be quite irregular, and even with 
them it is generally none too smooth. The pole-pieces of 
alternators are very often similarly rounded off or chamfered 
away for the same purpose. 

Nearly all modern alternating windings are like those just 
indicated, of the drum type. The Gramme winding is seldom 
or never employed, as it is hard to wind and repair and has, 
for alternators, no compensating advantages. Nor has the 
flat coil winding without iron core found a permanent place in 
American practice, although it is somewhat used abroad. 
There is considerable likelihood of eddy currents in the arma- 
ture conductors of such machines unless they are individually 
very thin, and for this and obvious mechanical reasons Ameri- 
can designers have adhered to the iron-clad armature, which 
is admirable mechanically and magnetically, and have taken 
other means to escape the difficulty of its high inductance. 

As in other dynamos, the theoretical E. M. F. generated by 
an alternator depends on the strength of the magnetic field, 
the number of armature conductors under induction, and 
the speed at which they are driven through the field. As an 
alternator receives load the E. M. F. at its terminals is reduced 
by three several causes. 

First, there is a loss of voltage due to energy lost in the 
armature conductors. This depends simply on the current 
and resistance and is numerically equal to C R. 

Second, there is self-induction in the armature windings, 
which, as we have already seen, involves an inductive E. M. F., 
lagging 90° behind the impressed E. M. F. The effect of 
this is to partly neutralize the impressed E. M. F., as in all 
cases of inductance. The amount of this disturbance depends 
on the frequency and the magnetic relation of the armature 
coils to each other and to the field magnets. This relation 
of course varies according to the relative position of the arma- 



TRANSMISSION BY ALTERNATING CURRENTS. 165 

ture teeth which carry the coils. In Fig. 72, purposely shown 
with somewhat exaggerated teeth, the armature is in the 
position of minimum inductance, for the magnetic field set 
up by the armature coils is not here much strengthened by 
the presence of the pole-pieces. If, however, the armature 
were shifted forward or backward so that each tooth would 
be just opposite a pole-piece, the field from the armature 
coils would traverse an almost complete loop of iron and the 
inductance of the armature would be a maximum. In this 
position the armature teeth might be almost as good magnet 
poles as the field poles themselves; at all events, consecutive 




Fig. 72. 



teeth would be united by an almost continuous iron core, and 
the armature inductance would be very high. 

One of the best ways of reducing this inductance and its 
train of troubles is to make the magnetization due to the field 
magnets as strong as is practicable. This not only utilizes the 
iron of the field magnets and armature to the best advantage, 
but, so to speak, preempts its power of receiving magnetiza- 
tion so that the current about the armature teeth finds a poor 
field for its inductive operations. In addition, this strengthen- 
ing of the field enables the required E. M. F. to be obtained 
with fewer turns per tooth. This of itself is a great advan- 
tage, since increasing the number of turns in an iron-cored 
coil runs up the inductance with appalling rapidity. A glance 
at Fig. 73 will show the reason why. Suppose we have a 
looped iron core wound with four turns of wire, a, h, c, d. If 
we pass a certain alternating current around two turns, a and 



166 



ELECTRIC TRANSMISSION OF POWER. 



h, we shall have a certain inductance due to the reaction of 
the change in magnetism on these two coils. Now, pass the 
same current around all four coils. The magnetization will 
be approximately doubled and the number of turns on which 
it acts will also be doubled. That is, each coil is acted upon 
by double the force and there are twice as many total coils. 
Hence, the total inductance will be about four times as great 
as at first, and in general it will increase with the square of 
the number of turns. If, however, as just suggested, the 
core is nearly saturated already, adding the two extra turns, 
c and d, will not anywhere nearly double the magnetization^ 




Fig. 73. 



since iron already magnetized responds less and less to addi- 
tional magnetizing force as this force increases. 

Hence, diminishing the number of armature turns that can 
act conjointly in producing effective magnetization lowers the 
inductance very rapidly. 

The third disturbing cause which tends to reduce the 
effective E. M. F. of an alternator is the reaction of the 
armature current, through the resulting magnetization, on 
the field magnets. We have already seen that when a closed 
coil is driven into and out of a magnetic field the induced 
current is always in such direction as to cause work to be 
done in driving the coil. But, since the current due to enter- 
ing the field is equal and opposite to that produced in leaving 



TRANSMISSION BY ALTERNATING CURRENTS. 167 

the field, the total magnetizations due to these currents are 
equal and opposite, and if one opposes the field due to a pole- 
piece the other will in an equal degree strengthen that field. 
Hence, provided these two actions are applied alike; i.e., 
are symmetrical with respect to the field, the total effect of 
armature current will be neither to weaken nor strengthen 
the field. 

In practice the effect of the armature reaction is two-fold. 
If the current be nearly in phase with the E. M. F. the main 
result of the magnetic field set up by the armature is to 




Fig. 74. 



distort that due to the field without greatly weakening it as 
a whole. The result of this distortion is that the E. M. F. 
does not increase and decrease steadily following a sine wave, 
but becomes irregular. The working E. M. F., as measured 
on a voltmeter, changes but a trifle, but the maximum 
E. M. F. becomes subject to great variations. Fig. 74 shows 
in a very striking manner the result of field distortion from 
a purely non-inductive load. Here a is the E. M. F. curve 
on open circuit and b is the curve as modified by the armature 
reaction at nearly full load. The arrow shows the direction 
of rotation of the armature. In this case the maximum 



168 ELECTRIC TRANSMISSION OF POWER. 

voltage was increased about 30 per cent, while the measured 
voltage was nearly constant. Bearing in mind that the 
E. M. F. at any moment is due to the rate of change of the 
magnetic induction through the armature, and not to the 
absolute amount of that induction, it is tolerably obvious that 
the effect of field distortion due to armature reaction may 
vary widely according to the shape and position of both 
the pole-pieces and the armature teeth. It may increase the 
maximum voltage as above, or decrease it fully as much, but 
if it is of any considerable magnitude it always deforms the 
E. M. F. wave very materially. 

If, however, through armature inductance or inductive load 
the current lags behind the E. M. F., we have a very different 
state of affairs. The current reaches its maximum after the 
armature coil has passed beyond the position of maximum 
E. M. F., and the net magnetization produced by it chokes 
back the field, at the same time greatly distorting it. 

If the only effect of armature reaction and inductance were 
to cause a loss of voltage there would be little cause for alarm. 
But as shown in Fig. 74, the E. M. F. wave-shape often un- 
dergoes profound changes, which may greatly increase the 
chance for serious resonance. As already noted, alternating 
generators, monophase and polyphase alike, give in practice an 
E. M. F. wave which is not sinusoidal, but contains the odd 
harmonics of the fundamental frequency. These are a neces- 
sary result of the variations in magnetic reluctance and arma- 
ture reactance when the armature is in various angular 
positions, as well as of subsidiary reactions in transformers 
and other apparatus. The harmonics of even order do not 
appear, since, unless a machine is deliberately made unsym- 
metrical, all the variations in E. M. F. are complete within 
each half period, the second half of the cycle merely showing 
a reversal of sign. Hence, only those harmonics appear which 
are themselves symmetrical with respect to a half period of 
the fundamental, i.e., by construction all the harmonics are 
of odd order. These harmonics have a very real existence, 
and can readily be identified by testing electrically for reso- 
nance, or even by hunting for them with a telephone in some 
cases. By taking the wave form of the machine by the con- 



TRANSMISSION BY ALTERNATING CURRENTS. 169 

tact method or photographically, the nature and magnitude of 
the harmonics are at once made evident. 

Fig. 75 shows the wave form of a machine that was carefully 
studied by Steinmetz. It is from a three-phase generator hav- 
ing but one armature tooth per phase per pole, and giving 150 
KW at 2,000 volts and 60 -'. Curve A is the E. M. F. wave of 
one coil to the common connection, at no load, B is the wave 
as calculated from a summation of the harmonics up to the 
fifteenth, and C shows the residual traces of still higher har- 
monics. To reduce the vertical scale to primary volts, mul- 



no 
















r-*, 


n 












s 
































/ 














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i 








/ 




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100 














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s>. 




/ 








i 














80 
70 
60 
































\ 














































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yfe 






















\ 






















cy 


























V 






















% 


























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80 
20 








> 


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\ 














y 


































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^ 















c 








^ 


■N. 








,f^ 






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=:i 






10 


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->?t- 


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btro. 


Y>. 


b 


IQH 


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d 


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30 


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GO 


60 




70 


80 

Fig 


«o 

. 75 


100 




10 


IM 


130 


lit 


1 


SO 


160 


170 


UT 



tiply by 10. Analysis of this wave showed that it corre- 
sponded approximately to the following equation: 

Sin a - .12 sin (3a - 2.3) - .23 sin (5a - 1.5) 
+ .134 sin (7a - 6.2). 

In other words the third harmonic has about 12 per cent, the 
fifth about 23 per cent, and the seventh about 13 per cent of 
the amplitude of the fundamental. 

At full load the shape of this wave is changed in a most sin- 
gular manner. The armature reaction shifts the magnitudes 
and positions of the variations in the magnetic field and of the 
harmonics due to them. Fig. 76 shows the wave form from 



170 



ELECTRIC TRANSMISSION OF POWER. 



this machine under load. The central depression of Fig. 75 
is replaced by a slight hollow between a high peak and a shoul- 
der, and the wave is conspicuously unsymmetrical, as might 
readily be predicted from the general effect of the armature 
reaction. The approximate equation to the wave of Fig. 76 is 



Sin a - .176 sin (3a + 11.7) - .085 sin (5a 
+ .01 sin (7a + 26.6). 



33.8) 



The effect of the armature reaction due to load has been 
greatly to strengthen the third harmonic, greatly to weaken 
the fifth, and nearly to suppress the seventh. 



HI) 

130 
120 






■ 






n 










V 
























~" 


^ 






















\ 




















































\ 


























100 

: 

70 

60 




























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; 




















\ 






























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V 




























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\ 




















































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10 



-10 

-20 














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V 




















/ 






























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y 






































s^ 




> 


/o 


10 


2 





30 


10 


50 


6 




70 


80 


90 


1 





10 


m 


130 


1 





150 


100 


17 


* "" 




/ 








_ 




_J 












u 


_ 


_ 




_ 










_ 




_ 





Fig. 76. 



Obviously changes of this sort may have a very great effect 
in the matter of resonance. Suppose, for example, that the 
conditions on the line at light load were such as to give 
marked resonance with the seventh harmonic of the frequency. 
Now, under all ordinary working conditions this harmonic 
would be practically absent; but if a large part of the load were 
thrown off, resonance would suddenly appear, and with the 
lessened armature reaction the general voltage would rise 
sharply, so that serious results might follow. In case of a high 
voltage generator, say for 10,000 volts, having the curves just 
given, at load the seventh harmonic would only have an ampli- 



TRANSMISSION BY ALTERNATING CURRENTS. 171 

tude of about 100 volts, while this amplitude would suddenly 
rise to 1,340 volts, increased perhaps four or five times by 
resonance, when the load was thrown off. Under other con- 
ditions throwing on load might produce an equally unpleasant 
effect. 

Lest it should be supposed that these wave distortions with 
the presence of strong high harmonics are extraordinary and 




Fig. 77. 

of merely theoretical importance, examples of such action 
from recent machines of first class make, obtained in commer- 
cial service, are here given. Fig. 77 shows the E. M. F. curve 
from a 750 KW, 5,500 volt engine-driven three-phaser, dis- 
torted by the presence of a strong thirteenth harmonic. The 
generator had a monodontal winding which is prone to give 
lower harmonics^ but these higher ones were mainly due to 




Fig. 78. 



the disturbing effect of a synchronous motor load. Fig. 78 
shows E. M. F. and current waves from a 1,500 KW three- 
phase turbo-generator, also on a synchronous motor load, and 
displaying conspicuous harmonics of the twenty-third order, 
in this case not traceable to the nature of the load, but structural 
and merely aggravated by the running conditions. The gen- 
erator had four slots per phase per pole in this case, but the 
magnetic density in the teeth was rather low. Under ordinary 



172 ELECTRIC TRANSMISSION OF POWER. 

circumstances these harmonics are probabl}^ quite harmless, 
but their frequency is so great that they might easily cause 
serious results with but a very moderate amount of capacity in 
the system. The curves shown are from oscillograph curves, 
reported by Dr. W. M. Thornton to the British Institute 
Electrical Engineers, and are quite sufficient to prove the im- 
portance of the subject. 

Such eccentricities can be avoided by scrupulous care in 
design, at least for the most part, and should be eliminated 
from, every machine used upon circuits where by reason of 
unusual length, or the presence of cables, there is danger of 
resonant effects. At the voltages now generally used for power 
transmission, insulation is difficult enough without incurring 
the risks that come from preventable dangers of this sort. 

The magnetizing and demagnetizing effects of the arma- 
ture current in case of inductive load no longer can balance 
each other, for they are unsymmetrical with respect to the 
poles. If the angle of lag is large the result will be a very 
serious weakening of the field, and a correspondingly large 
drop in the effective voltage. For example, a certain alter- 
nator of 120 KW output has 40 turns of wire per armature 
tooth, carrying a normal full load current of 60 amperes. 
There is thus a possible demagnetizing force of 2,400 ampere- 
turns at full load. The ampere-turns per pole-piece in the 
same machine are 3,600, so that if the current should lag 
enough to give the armature reaction full play, as might 
happen from excessive armature inductance alone, the total 
net magnetizing force would be reduced to a third of its nor- 
mal amount and the resulting voltage to a half or less. It 
is in fact common enough to find alternators that require 
from 50 to 100 per cent increase in the exciting ampere-turns 
to hold them at normal voltage under a full-load current 
lagging even 15° or 20°. 

Between inductance and armature reaction the effective 
E. M. F. of alternators generally falls off rapidly under load 
unless special care be taken with the design. The loss from 
ohmic resistance is usually trivial compared with those just 
named. It is, in fact, perfectly practicable to build an alterna- 
tor with inductance and armature reaction so exaggerated, 



TRANSMISSION BY ALTERNATING CURRENTS. 173 

that a very slight increase in current will cut down the voltage 
so rapidly as to keep the current virtually constant. This plan 
was successfully carried out in the remarkable Stanley alter- 
nating arc machine of a few years ago. 

In this case the current varied only about 10 per cent, while 
the voltage varied between a few volts and over 2,000. An 
automatic short-circuiting switch was provided to avert dan- 
gerous rise of voltage in case of an accidental open circuit. 

In so-called constant potential alternators, as usually built, 
the inherent regulation is by no means good. Fig. 79 gives 
an excellent idea of the performance of some of the earlier 
machines in this respect, and it is about what one would find 



130 



120 



100 



90, 



10 



30 



50 



60 



70 



AMPERES 
Fig. 79. 



in many alternators now in service, except for their compound 
winding. 

It has often been held that high inductance and large arma- 
ture reaction are desirable in alternators in order to prevent 
burn-outs in case of accidental short circuits. While it is per- 
fectly true that sufficiently crude armature design does produce 
this effect, by limiting the possible current, it is equally true 
that a machine with sufficient inductance and reaction to serve 
as a practical safeguard will regulate so atrociously as to be 
under many circumstances incapable of decent commercial 
service under present conditions. When it was sufficient for 
an alternator to give current that with sufficient hand regula- 
tion could supply house to house transformers most of the 



174 



ELECTRIC TRANSMISSION OF POWER. 



time, high inductance machines, which are easy and cheap to 
build, answered the purpose. 

At present, when the importance of good regulation is gener- 
ally understood, and most large alternating plants must look 
forward to assuming a motor load, low inductance machines 
with small armature reaction are essential for first-class service. 
For power transmission plants with heavy mixed loads of 
lights and motors, no other class of machine should be toler- 
ated, or can be used without incessant annoyance. 

Most even of the older alternators are compound-wound 
to compensate for armature effects, and are thus enabled to 
work successfully up to outputs at which the voltage begins to 



laoo 
















NON- 


INDU( 


:tive 


LOAD 










^^ 


^ 








INDUCTIVF 


to^ 




noo 








^ 


^ 












■^ 


^ 


^ 


^ 




















1000 


^^ 

















































50 75 100 

KILOWATTS OUTPUT 
Fig. 80. 



135 



150 



fall off too fast to be thus compensated. So long as the com- 
pounding process actually gives good regulation, it is useful 
and enables the generators to be worked at a high output. As 
a matter of fact when used with generators of the older type, 
even compounding left much to be desired. As alternating 
practice has gradually improved, compound-wound alternators 
have been more skillfully designed, and recent machines give 
on non-inductive load a very fair approximation to constant 
potential. Fig. 80 shows the E. M. F. of a modern over-com- 
pounded alternator at varying load. If, however, the current 
has even a moderate lag behind the E. M. F.. owing to induc- 
tance in the machine or the load, the machine will no longer give 
constant potential, and the voltage may fall off rapidly as the 



TRANSMISSION BY ALTERNATING CURRENTS. 175 

load comes on, as shown in the cut. The reason for this we 
have already found in the extra increase of field excitation 
necessary to compensate for the demagnetizing effect of arma- 
ture reaction. Incidentally if the current commuted to supply 
the series field lags much, the process of commutation cannot 



LU 



A A 



r^_ 



I 







D 



Fig. 81. 



go on normally without adjusting the brushes to compensate 
for the lag. 

Therefore, for inductive load the compounding has to be 
greatly increased, and even then is correct only for a particu- 
lar inductance. 

It must be understood that alternators are compounded on 
the same general principles as continuous current machines, 
except that instead of the current for the series winding being 
derived from the general commutator of the dynamo, it is 



176 



ELECTRIC TRANSMISSION OF POWER. 



generally obtained from a simple special commutator. A 
shunt around this commutator diverts most of the main cur- 
rent, while a portion is rectified and passed around the fields. 
Fig. 81 shows in diagram a common compounding arrange- 
ment. The two collecting rings A and B with the commutator 
C are mounted on the armature shaft. Brushes on A and B 
take off the alternating current. One of these rings, A, leads 
directly to line. The current going to the other ring is divided, 
part passing around C through the resistance box D, and part 
being rectified by the commutator for use in the series field. 
This commutator has as many segments as there are pairs of 
poles in the field, the alternate sections being electrically united. 



2400 



O2300 

> 

>- 

oe 

^ 2200 



2100 



2000 



o 

_i 

_j 
_j 



10 



20 



30 



40 50 60 

OUTPUT IN K.W 

Fig. 82. 



ro 



80 



90 



100 



By varying the resistance D, the amount of current diverted 
into the field can be varied, and the compounding may thus be 
arranged to keep the voltage constant at the terminals or at 
any point on the line. A similar change in D may be made 
to adjust the compounding for inductive load of any given 
power factor. 

For non-iuductive loads, or for inductive loads of constant 
power factor, this compounding gives good results, but for a 
load of widely varying power factor it is nearly worthless 
unless supplemented by hand regulation. 

If compounding is to be successfully used for keeping con- 
stant potential on a circuit of lights and motors subject to 
considerable variations in the power factor, it must be applied 
to a generator of very low inductance and armature reaction. 



TRANSMISSION BY ALTERNATING CURRENTS. 177 

Otherwise no adjustment of the compounding for any particu- 
lar power factor will give approximately constant potential 
when the power factor varies. 

For example it would be hopeless to attempt to compound 
in the ordinary way an alternator having a characteristic 
like Fig. 79, so that it would be tolerable on a commercial cir- 
cuit of lights and motors. On the other hand, a generator 
having a voltage characteristic like Fig. 82 could readily be 
so compounded. Here the fall in voltage at constant field 
excitation, from no load to full load (non-inductive), is about 
3i per cent. Under inductive load this fall would be in- 
creased considerably, but from the usual ratio of inductive 
drop to armature reaction found in the best modern gener- 
ators, the variation for the power factors hkely to be encoun- 




FlG. 83. 



tered with a mixed load would be somewhat smaller than the 
original drop. The total variation from no load to full in- 
ductive load would then be between 6 and 7 per cent, and 
with compounding adroitly adjusted for average conditions 
the greatest variation from normal voltage could easily be 
brought within 2 per cent. A little intelligent hand regu- 
lation at certain times of the day would improve even this 
good result. 

These considerations apply to polyphase as well as to mono- 
phase generators. The advent of polyphase work has done 
much to improve all alternators, and especially with respect to 
regulation. 

The generation of polyphase alternating currents is a very 
simple matter. The object in view is the production of two or 
more similar currents differing in phase by some convenient 



178 ELECTRIC TRANSMISSION OF POWER. 

amount, usually 60° or 90°. To obtain two currents 90° apart 
in phase, it is only necessary to clamp together the shafts of 
two common alternators, so that, for a construction like Fig. 
70, the slots of one armature would be opposite the teeth of 
the other armature. The armatures would then give currents 
90° apart in phase. Such combination alternators were built 
for the Columbian Exposition by the Westinghouse Company, 
and were used for the principal lighting and power circuits. 
These structures are, however, expensive for the output obtained, 
and the two windings are nearly always put on a single arma- 
ture core, and spaced as just described. Fig. 83 shows dia- 
grammatically a winding of this nature. There are four times 



Fig. 84. 

as many armature slots as there are field poles. Each coil 
spans two teeth. The coils shown by solid lines form one phase 
winding, the dotted coils the other phase winding. Each set 
of coils is connected as an ordinary monophase winding, and 
the terminals are brought out to two pairs of collecting rings. 
Such a winding gives two simple alternating currents related 
in phase as shown in Fig. 84. The armature core is very fully 
occupied by the two windings, rather more advantageously 
than it could be by a single winding, so that the machine gives 
a somewhat better output as a two-phaser than would be 
possible with a simple alternator of the same dimensions. 
And, what is of more importance, the regulation of the machine 
as a two-phaser is much better than it would be as a single- 
phaser. In the first place the armature inductance is greatly 
reduced by the distribution of the windings and the reduction 
of the ampere-turns per armature tooth. Second, the same 



TRANSMISSION BY ALTERNATING CURRENTS. 179 



causes act to cut down the armature reaction in case of a lagg- 
ing current. Anything that improves the intrinsic regu- 
lation also means greater output for unimproved regulation. 
Moreover, the increased number of armature teeth gives a 
more uniform reluctance than in the case of fewer teeth, and 
hence tends to give a better approximation to a sinusoidal 
wave form. 

So, aside from the value of polyphase currents for motor pur- 




FlG. 85. 

poses, which we shall presently examine, polyphase winding is 
valuable on its own account as increasing output and improv- 
ing regulation. In fact, diphase windings were devised for this 
purpose before their importance in the operation of motors 
became generally known. 

The value of a subdivided winding in reducing inductance 
and armature reaction was greatly emphasized by the intro- 
duction of polyphase generators, and it was a short step from 
monodonotal windings having one coil and virtually one tooth 
per phase per pole, to windings in which each phase winding is 
split up into several sets of coils in adjacent slots, thereby 
still further decreasing the effective inductance and armature 
reaction. Such windings may be called polyodontal, from their 
several teeth per phase per pole, and are very generally used in 



180 ELECTRIC TRANSMISSION OF POWER. 

the best recent machines. A fine example of this class of 
winding is shown in Fig. 85. This is a quarter section of the 
armature of one of the original 5,000 HP Niagara generators, 
showing a portion of one coil belonging to a single phase. 
The full winding is composed of two conductors per slot, half 
the total slots, in alternate groups, belonging to each phase. 

Such complete subdivision of the coils results in low induc- 
tance and a very low armature reaction. A similar winding 
could be used for a monophase generator, and will have to be 
employed if monophase machines come to be used extensively 
for power transmission purposes. The form of armature slot 
used for polyodontal windings is shown in Fig. 86, a single 
segment of one of the core plates of the armature of the Niagara 
two-phaser. The appearance of one of these great machines 




Fig. 86. 



complete is admirably shown in the frontispiece, showing the 
interior of the Niagara station. The field magnets are re- 
volved instead of the armature, although they are exterior to 
it. A very powerful fly-wheel effect is gained by this arrange- 
ment, since the weight of the revolving structure, turning at 
250 r. p. m., is about 75 tons, half of this being in the field 
itself. This is about 12 feet in diameter, a single forged steel 
ring with twelve massive pole-pieces secured to its inner face. 
The normal voltage of the machine is about 2,250, and the 
frequency is 25^. The stationary armature is provided with 
six ample ventilating ducts, through which air is forced by 
the revolving field. Fig. 87 shows a vertical section of the 
whole apparatus with its shaft and upper bearings. A hun- 
dred and forty feet below the generator is the turbine which sup- 
ports by hydraulic pressure the weight of the revolving mass, 



TRANSMISSION BY ALTERNATING CURRENTS. 181 

save a ton or two of residual weight, which may be either posi- 
tive or negative, and which is taken care of by a thrust bearing. 
The full load of this generator is 775 amperes on each of the 
two circuits, and at this load the commercial efficiency is 
nearly 97 per cent — a figure very close to the possible max- 
imum. The exciting current for the fields is derived from a 
rotary transformer, and is led into the revolving magnets 
through a pair of collecting rings shown in Fig. 87 at the 




Fig. 87. 

extreme top of the shaft. The armature current is of course 
taken from stationary binding posts. Altogether this Niagara 
machine was a fine specimen of polyphase construction. 

When three-phase currents instead of two-phase are to be 
generated, separate armatures are out of the question, and a 
winding similar to that of Fig. 83 is frequently employed. To 
obtain the three currents, however, three separate windings 
are employed, arranged as in Fig. 88. The coils are connected 



182 



ELECTRIC TRANSMISSION OF POWER. 



so that a, a, a, etc., form one phase winding, h, b, etc., a second, 
and c, c, etc., the third. The close similarity of this winding 
to the two-phase shown in Fig. 83 is at once apparent. 

It is worth noting that these three windings are spaced 60° 
apart, instead of 90°, as in a winding for two phases. Natur- 
ally, therefore, the currents generated would be different in 




Fig. 88. 



phase by only 60°, giving the arrangement of currents shown 
in Fig. 89. This is homologous with the two-phase current 
system of Fig. 84. 

In practice it is necessary, however, to have the sym- 
metrical arrangement of phases given by three similar cur- 




FlG. 89. 

rents 120° apart. This is very easily obtained in the external 
circuit by winding one set of the armature coils in a direc- 
tion reversed from the other two, or by merely reversing the 
terminals in making connections. The result of this is a true 
three-phase current, such as is shown in diagram in Fig. 90. 



TRANSMISSION BY ALTERNATING CURRENTS. 183 

It has now the curious property that at all times the system is 
simultaneously carrying currents substantially equal in both 
directions, as will readily appear from inspection of the curves. 
With such a current it is usual to combine the circuits cor- 
responding to the several armature windings. Otherwise we 
would be compelled to deal with circuits of six wires, and the 
generator would have six collecting rings. 

Moreover, the distribution circuits formed by combining 
the circuits as just indicated have the advantage of economy 
in copper, as we shall presently see. Hence, the three-phase 
system has become the mainstay of electrical power transmis- 
sion so far as the principal circuit is concerned. The genera- 




FlG. 90. 



tors may be two-phase and the distributing circuits two-phase 
when convenience dictates, but the main line is, save in very 
rare instances, worked three-phase. The change from two- 
phase to three-phase, or the reverse, is accomplished in a beau- 
tifully simple and efhcient manner, to be described later, 
lender certain circumstances the use of a two-phase generator 
has at least the theoretical advantage that the currents in 
the respective armature windings, being in quadrature, can 
have little or no mutual reaction, so that the two phases are 
more independent than the three phases of a three-phaser. 

As might be expected, the subdivision of windings in a three- 
phase armature results in small inductance and armature reac- 
tion, smaller in fact than would be found in a similar two-phase 
winding. Nevertheless, experience shows that if the armature 
has only a single coil per phase per pole, the reaction is too 
great for first-class regulation, and the curve of E. M. F. is 



184 



ELECTRIC TRANSMISSION OF POWER. 



rather too wdde a departure from the sine wave. It is quite 
usual, therefore, to adopt the polyodontal construction with 
from two to four coils per phase per pole. A machine carefully 
designed on these lines can be made to give excellent regulation, 
with voltage not varying more than 3. or 4 per cent from no 
load to full non-inductive load, and is capable of giving a very 
close approximation to a true sinusoidal wave, a valuable 
characteristic for longidistance transmission. Fig. 91 shows 
the wave form given by one of these polyodontal three-phasers. 



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The full curve shows the actual E. M. F., the dotted line the 
corresponding sine curve, and the irregular line at the base of 
the figure the difference between the two. 

There are several methods of connecting a three-phase wind- 
ing to its external circuit. The two chiefly used are generally 
known as the "star" and ''mesh" connections. In the former, 
one end of each of the three windings is brought to a common 
junction, and the three remaining ends are connected to three 
line wires. The three lines then serve in turn as outgoing and 



TRANSMISSION BY ALTERNATING CURRENTS. 185 

return circuits, the maximum current shifting in regular rota- 
tion from one to the others in succession. The three E. M. F.'s 
in the three coils differ in phase by 120°, owing to the reversal 
of which we have spoken. We may draw the star connection 
diagrammatically in Fig. 92, drawing the three coils ah c 120° 
apart to show the relation of the E. M. F.'s and currents, 

A 




Fig. 92. 

although they lie on the armature as shown in Fig. 88. Three 
of the terminals meet at the point o, the others are connected 
respectively to the lines A, B, C. As the three windings on the 
armature are alike, the E. M. F.'s generated by the three coils 
are equal. So if each winding a, h, c, is designed for 1,000 
volts, that will be the voltage between the point o and each of 
the three Hues A, B, C. Clearly, however, the voltage between 




Fig. 93. 



any two of these lines, as A and 5, is a very different matter, 
since it results from the addition of the voltages of a and h, 
which are, however, 120° apart in phase. They must then be 
added geometrically. Now the chord of 120° is V3 times the 
radius, so that the geometrical sum of the voltages a and h, 
120° apart, is 1.732 times either of them. The voltages then 



186 ELECTRIC TRANSMISSION OF POWER. 

between A and B in the case in hand aggregate 1,732. The 
same is evidently true of the other pairs of hnes B, C, and C, A. 

The other ordinary three-phase connection is the mesh, in 
which the six terminals of the three coils are united two and 
two, and the lines are connected to the three points of junc- 
tion. This arrangement is shown diagrammatically in Fig. 93. 
Here each coil must generate the full E. M. F. between any 
two of the lines, but the current in any line, as B, is made up of 
the geometrical sum of the currents in a and 6, differing in 
phase, just as the E. M. F. between lines in Fig. 92 was made 
up of the sum of two E. M. F.'s. The current in 5 being then 
so constituted, is V3 times the current in a or b, and so on for 
the other lines. In the mesh connection we deal with resul- 
tant currents just as in the star we find resultant E. M. F.'s. 

An armature designed for a given working voltage, measured 
in the ordinary way between lines, would, if planned for star 
connection, have fewer turns of larger wire than if intended for 
mesh connection. This is sometimes convenient, and is useful 
in keeping the voltage between coils low. The mesh connec- 
tion on the other hand has more turns of smaller wire, as the 
current is diminished Avhile the E. M. F. in each coil is the full 
E. M. F. between lines. This property is useful under certain 
conditions, as it makes the E. M. F. between any two lines 
somewhat less dependent on the actions going on in the other 
pairs of lines. The same windings can of course be connected 
either star or mesh, according to the dictates of convenience. 
Both these combination circuits have in common one immensely 
valuable property. They require for the transmission of a 
given amount of energy at a given percentage of loss, only 
75 per cent of the weight of copper required for the same trans- 
mission at the same working voltage, by continuous current 
or by any alternating system having two wires per phase. 
That is, if 100 tons of copper are required for a given transmis- 
sion by continuous current, single-phase alternating, two- 
phase with two circuits, or three-phase with three circuits, 
75 tons will suffice for the same transmission by the star or 
mesh three-phase circuit without any increased loss of energy. 
The proof of this saving is very simple. Assume a three-phase 
circuit carrying a non-inductive load at V volts between lines, 



TRANSMISSION BY ALTERNATING CURRENTS. 187 

the current in each Hne being I and the resistance r. Then for 
a star connection, as we have already seen, the voltage in 

each branch to the neutral point o (Fig. 92) is V — -= , the cur- 

Vs 

rent in each branch is /, the power in each branch is — ;= IV, 

V3 

and the total power is IV v3. 

The loss in each branch of the circuit is obviously Pr, and 
the total loss for the above power 3Pr. Now let the same 
amount of power be transmitted by a single-phase circuit at 
the same voltage V. The current will evidently have to be 
/ v3. Let / be the resistance of one of the two monophase 
wires, such that the total loss shall be 3Pr as before. The 
resistance of the complete circuit will be 2r', and the total loss 
6Pr'. But since 

QPr' = 3Pr, 



That is, the resistance of each of the monophase wires must be 
only one-half the resistance of a single three-phase wire. The 
cross section of each monophase wire must then be double the 
cross section of one three-phase wire. If the weight of the lat- 
ter be w, the total weight of the three-phase copper will be 
Sw, while the weight of the two monophase leads of double 
cross section will evidently be 4:W for a circuit of the same 
length. A mesh connected three-phase system leads to ex- 
actly the same result, since the voltage in each branch is V 

(see Fig. 93), the current is — — , the power per branch — -IV, 

V3 V3 

the total power IV V3, and the loss oPr, as before. 

The result seems so singular that in the early days of the 
three-phase system it was slow to be accepted by the public, 
until checked experimentally with the greatest precision, and 
by various experimenters. A similar saving can be effected 
by the use of some other polyphase combination circuits, but 
it happens that the three-phase combination is the one least 
open to practical objections. 



188 



ELECTRIC TRANSMISSION OF POWER. 



In actual working the two-phase system is nearly always 
installed with a complete circuit per phase as regards the dis- 
tribution circuits, unless for short connections to apparatus; 
the three-phase system is used with the star or mesh combina- 
tion, except for occasional special work, and the more compli- 
cated polyphase systems are practically not used at all, save 
that in working rotary converters, the final connection is some- 
times with four, six or even more phases to take better advan- 
tage of the armature winding. 

In speaking of the voltage of an alternating circuit, it must 
be borne in mind that we do not mean the voltage correspond- 
ing to the extreme crest of the E. M. F. wave, but that vol- 
tage which, multiplied by the current in a non-inductive circuit, 
equals the energy in that circuit. This effective working vol- 
tage bears no fixed relation to the real maximum voltage, since 




Fig. 94. 



their ratio evidently varies with the shape of the E. M. F. wave. 
For a sine wave the ratio is 1.414, so that an alternating working 
pressure of 1,000 volts means a maximum voltage of 1,414. 
As may be judged from Fig. 91, this ratio is very nearly true 
for the best modern alternators. 

Save in rare instances the work of power transmission is 
done by two-phase or three-phase currents. Abroad some pure 
single-phase plants are in operation with fairly good results, 
but the difficulty of getting good single-phase motors has so far 
rather checked development along this line. 

In this country, a decade since, the so called '' monocyclic" 
system, now obsolete, was introduced in a few plants where the 
motor load was merely incidental to lighting. 

In this system there was a main armature winding to which 
the lighting circuit was connected as in ordinary single-phase 
working, while a subsidiary armature winding furnished mag- 



TRANSMISSION BY ALTERNATING CURRENTS. 189 

netizing current for the motors. The general arrangement 
of the armature coils is shown in Fig. 94. The winding in 
the small intermediate slots was of the same size of wire as 
the main coil, but had only one-fourth as many turns, and 
consequently one-quarter the main E. M. F. This so-called 
'Heaser" E. M. F. was obviously 90° in phase from the main 
E. M. F. The relation of the two E. M. F.'s is better shown 
in Fig. 95, where A B C is the main E. M. F. and B D the teaser 
E. M. F. The generator had three collecting rings, of which 
the middle one was connected to D. The outer rings had the 
full E. M. F. between them, while between D and C the E. M. F. 
was the geometrical sum oi B C and B D, approximately 
.56 of the main E. M. F. For motor service the resultant 
E. M. F.'s differing in phase were variously combined, usually 
into approximately three-phase relation, although in normal 



^ B 

Fig. 9 



running all the currents in the motor remained in very nearly 
the same phase. The object of this system was to obtain for 
lighting purposes a perfectly simple circuit, the voltage of 
which should be quite undisturbed by actions going on in the 
subsidiary motor circuit, which object was attained if the 
generator was so arranged as to hold its voltage closely under 
inductive load. 

, A similar device for simplifying the operation of lighting 
circuits is a three-phase system arranged to supply the entire 
lighting service from two of its lines, as A and B, Fig. 92. 
The other two connections B C and A C would only be used 
for motor service, and if desirable the coils h and c could take 
up very little space on the armature. Still another of these 
heterophase schemes employs regular single-phase alternators 
for the lighting work, and a small adjunct machine in phase 90° 
from the others, and connected with them to form a two-phase 



190 



ELECTRIC TRANSMISSION OF POWER. 



circuit with one common wire. This connection is used for 
starting ordinary two-phase motors. 

In general, the heterophase systems have no substantial 
advantage over the ordinary polyphase systems, and are rarely 
employed. In the chapter on centres of distribution, the 
working properties of various alternating systems will be 
taken up in more detail. 

In general construction and arrangement of parts all alter- 
nators are similar. Those specially intended for power trans- 




FlG. 96. 



mission are sometimes, however, modified for convenience in 
obtaining high voltage or for direct coupling to water wheels. 
The vertical shaft arrangement as exemplified in the original 
Niagara machines is now and then used both in this country and 
abroad. Machines for 3,000 to 5,000 volts and upward are 
best constructed with stationary armatures, to avoid mechan- 
ical strains on the high voltage insulation. In following this 
design the armature is usually exterior to the field magnets 
an it is indeed in the later generators of the great Niagara plant. 



TRANSMISSION BY ALTERNATING CURRENTS. 191 

It is very doubtful whether the fly-wheel effect gained by 
revolving an exterior magnet compensates for the great inac- 
cessibility of the high voltage armature. 

A characteristic example of the revolving field alternator is 
shown in section in Fig. 96.* It is a large polyphase generator 
for direct connection to a water-wheel, and the cut gives a good 
idea of its mechanical arrangement. The stationary armature 
is assembled on the interior of a supporting circular box girder 
cast in upper and lower halves. In the fourth quadrant of the 
cut this is seen in section, bearing dovetailed projections for 
supporting the laminae of armature iron. These are curved 
segments as shown in the third quadrant, twelve segments to 
the entire circle. In assembling the armature each layer 
breaks joints with the next, and when the whole mass of 
laminae is built up it is held firmly together by heavy end plates 
which are secured by bolts passing through the space left 
between the laminae and the supporting girder. This stage of 
the construction is seen in the second quadrant. Finally after 
the armature coils are in place they are protected by a seg- 
mental ventilated shield as seen in the first quadrant. The 
revolving field magnet is likewise built up of segmental laminae 
dovetailed to supporting castings, which are in turn carried 
by the two heavy steel plates which, bolted to the hub, form 
the driving spider. As in most such constructions the pole 
tips are of separate laminae dovetailed or interlocked with the 
laminae of the polar projections. The field coils are held in 
place by shoes and radial bolts to relieve the pole tips of the 
centrifugal stress. For lower speed machines the poles are 
often solid save for the dovetailed laminated tips, and are 
simply held to the rim of the field spider by radial bolts. 

The construction of such machines is very various, but the 
main point is that the high voltage windings are stationary, 
kept well clear of each other, and singularly accessible so that 
damaged coils are very easily replaced. Current is led to the 
field by two small slip rings. Even for low voltage niachines 
this construction is very generally preferred by reason of the 
greater security of the windings, and the absence of the large 
slip rings and their collecting devices. 

* See Trans. A. I. E. E. Feb., 1904 



192 



ELECTRIC TRANSMISSION OF POWER. 



Still another form of alternator in which the armature, 
and field windings as well, are stationary, is found in the ''in- 
ductor" dynamo. Of this the most familiar types are those 
introduced by Mr. Mordey in England and by Mr. Stanley in 
this country. In such machines the magnetic circuit through 
the armature coils established by fixed field coils is periodi- 
cally closed and opened by revolving pole pieces which them- 
selves carry no wire. The principle is illustrated in Fig. 97, a 




Fig. 97. 



cross section of the Stanley inductor dynamo. Here the cir- 
cular yoke C carries two rings of laminae each provided with 
windings, B, arranged much as in the alternator just described. 
Within these rings revolve two sets of laminated polar pro- 
jections borne on a massive spider which completes the mag- 
netic circuit. The stationary field winding A surrounds the 
spider as a whole without touching it. Evidently all the 
poles at one end of the spider are north poles and those at the 
other end south poles, and the armature coils are connected 




Fig. 1. 




Fig. 2, 



PLATE IV. 



TRANSMISSION BY ALTERNATING CURRENTS. 193 

accordingly. As a rule inductor dynamos are not economical 
of material owing to the nature of the magnetic circuit, and 
their gain in security and simplicity over a revolving field 
alternator of the ordinary sort is hardly enough to balance 
the disadvantage, so that they are now less used than formerly, 
although in themselves excellent machines. 

Plate IV shows the field and the two halves of the armature 
of a modern high voltage polyphase generator for direct con- 
nection to the prime mover. In this case the diameter of the 
armature frame is so great that it has been found desirable to 
design it as a hollow circular truss in order to give it the ne- 
cessary rigidity against distortion by its own weight and by 
inequality of magnetic pull, if there were a trifling eccentricity 
due to wear of the bearings. In some of the early machines 
of large diameter, flexure from the weight alone was very 
troublesome. Half the armature coils are shown in place and 
wedged in, and a coil belonging in the second half is all ready 
to put in place. Four shapes of coils are necessary to com- 
plete this winding, but they can be kept well clear of each 
other at the ends and are easy to put in and take out, so that 
in case of damage a coil can be easily replaced, although it 
may sometimes be necessary to move several others to get at 
the damaged one. An injured coil, however, can readily be put 
out of circuit by cutting it loose at the ends, insulating them, 
and connecting the adjacent coils of the same phase across the 
dead one. A generator so temporarily repaired in a few min- 
utes can be run imtil opportunity offers for permanent repairs, 
and can even be worked in parallel with others without material 
difficulty. 

To facilitate repairs the armatures of large revolving pole 
machines are often carried on a sliding bed, so that they can 
be shifted by their own width along the shaft, exposing the 
windings of both armature and field. 

The field is really a compact, massive fly-wheel with the 
poles bolted on its rim, the poles surfaces being shaped so as to 
give as nearly as may be a sinusoidal wave. The pole-pieces 
are generally laminated, at least near the tips, and are some- 
times provided with ventilating spaces like those in the ar- 
mature. 



194 



ELECTRIC TRANSMISSION OF POWER. 



The advantage of revolving field generators is so great in 
point of easy insulation and ready collection of even very great 
currents, that this type of machine has been rapidly displac- 
ing the older form for high voltage work, and indeed for large 
work of every kind. In such generators voltages of 10,000 
and 12,000 are now quite common, and the limit has not been 
reached. 

Fig. 98 shows the efficiency curve of one of the huge modern 
high voltage three-phasers. It is from a 5,000 KW, 11,000 
volt directed connected generator for the Interborough Rapid 
Transit Co., of New York City, and while it does not show the 







































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20 


30 


3000 
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t Ou;tpu 


5000 


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7000 



Fig. 98. 



small frictional and air resistance losses, is very striking as 
illustrating first, the veiy high efficiency reached by such 
machines, and second, the remarkably uniform efficiency at 
varying loads. The curve passes 94 per cent at a little below 
quarter load, reaches a full load efficiency of 98 per cent, and 
rises even slightly higher on a 25 per cent over load. The regu- 
lation of this generator is also excellent, being upon non-induc- 
tive load in the vicinity of 5 per cent. It is built with a revolv- 
ing 40 pole field 32 feet in diameter and the armature winding 
is distributed in four slots per phase per pole, each slot contain- 
ing three bars. 

In large polyphase generators the question of automatically 
regulating the voltage in response to changes of load is a seri- 



TRANSMISSION BY ALTERNATING CURRENTS. 195 

ous one, and no final solution of it has as yet been reached. 
It is not economical to build generators with so small inherent 
variation of voltage as is in itself desirable. In small poly- 
phase machines compounding has been accomplished with an 
arrangement of parts similar to that shown in Fig. 81, the 
connections being so modified as not to take the commutated 
current from a single phase. This is troublesome in machines 
requiring considerable energy for the field excitation, and be- 
sides it only compounds correctly for a particular value of the 
power factor, which in many plants is constantly changing. 
Several modern methods of compounding direct the com- 




FiG. 99. 



pounding at the exciter. A rotary converter is used as ex- 
citer, and the voltage at its commutator, which depends on 
the alternating voltage applied at the slip-rings, is modified in 
various ways in response to changes in the magnitude and 
phase of the working currents from the generator. A typical 
plan of this kind, successfully applied by the author some ten 
years ago, is shown diagrammatically in Fig. 99. Here the 
generator fields A^ A^ are fed from the commutator end of a 
rotary converter F F. Current from the main collecting rings 
a is led to the collecting rings h of the exciter through the re- 
active coils c c c on the cores M M M, which are also wound 
with series turns d d d in the main leads of the generator. At 



196 



ELECTRIC TRANSMISSION OF POWER. 



light loads the voltage at b is cut down by the reactance, while 
as the main current increases or lags the series turns d d d raise 
the voltage a b, and hence strengthen the generator field. By 
properly proportioning the coils c c c, d d d, and their cores 
M M M, the apparatus can be made to regulate the voltage 
very closely for all loads of the generator, inductive or non- 
inductive, or even may over-compound on inductive load so as 
to compensate for the change in the inductance of the system. 
Fig. 100 shows the working of this device when arranged to 
show extreme over-compounding on inductive load. The gen- 
erator chosen was one which uncompounded would drop its 
voltage about 40 per cent on a heavy inductive load. Curve 



150 



1A0 



5l30 
5 



120 
110 

















































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A shows the regulation of the secondary voltage on non-induc- 
tive load, curve B the over-compounding produced by a load of 
induction motors running light, having a power factor of not 
over 0.25. 

The same general principle has been lately applied in several 
forms with very promising results. An interesting modifica- 
tion is the compensated field alternator recently brought out 
by the General Electric Company, and shown in Plate V. 
Here the exciter armature is on the shaft of the main machine, 
and is in a field having the same number of poles, so that it 
revolves synchronously pole for pole with its generator. Ex- 
citer and main fields are fed in shunt from the exciter commu- 
tator, but the exciter armature also receives throTigh its col- 




Fig. 1. 




Fig. 2. 



PLATE V, 



TRANSMISSION BY ALTERNATING CURRENTS. 197 

lector rings an auxiliary current derived from series trans- 
formers in the main leads of the generator. This device holds 
the voltage with beautiful precision under ordinary changes of 
load and lag, but the necessity of being in mechanical synchron- 
ism is somewhat embarrassing, save in high speed machines. 
Another very pretty method of regulation by compounding 
the exciter, is that due to Prof. F. G. Baum* and shown dia- 
grammatically in Fig. 101. In this device a little generator of 
a few hundred watts capacity is mechanically driven in syn- 
chronism with the main generator G. Its fields A A' are 
excited by a few turns of the main generator current. The 




Fig. 101. 



armature B has a very simple winding, one terminal of which 
goes to a solid ring connected by a brush with the lead 6, the 
other goes to a pair of opposite segments about 90° wide and 
thence to the lead 6'. When the fields are excited from the 
main current and the armature is turning in synchronism, the 
machine evidently gives a partially rectified current, more or 
less of the waves of one polarity being bitten off short, accord- 
ing to the position of the segments of the divided ring. The 
oscillograph record of the resulting pulsatory current is shown 
in Fig. 102. Now if the current in the main line lags the 
effect is precisely the same as if the segmental ring had been 
turned forward a little, thus increasing the amplitudes of the 
* Trans. A. I. E. E. May, 1902. 



198 ELECTRIC TRANSMISSION OF POWER. 

peaks, and hence the effective E. M. F. of the pulsatory current. 
A leading current produces precisely the opposite effect. This 
automatically varied current excites the field of a second little 
machine C which compounds the exciter through the series 
coil D. The rheostats R R enable the compounding to be 
accurately adjusted. The effect of this apparatus is to regu- 
late not only for varying current but for variations of power 
factor, in a very satisfactory manner. It may also be applied 
to synchronous motors and rotary converters. 

Along such lines as these, good results are certainly attain- 
able, and in addition there are several automatic devices for 
working a rheostat i^ the generator field so as to hold the vol- 
tage constant, irrespective of load or lag. These with other 
regulating apparatus will be described in another chapter. 




Fig. 102. 

As a matter of fact, in much power transmission work com- 
pound winding is not necessary, since the machines hold their 
voltage closely without it if well designed, and in large plants 
the variations of load are usually so gradual that the voltage at 
the end of the transmission line can be easily kept constant 
by hand regulation. Again, in many transmission plants sev- 
eral lines are fed by one generator, so that no compounding 
would suit all the lines; and whenever a substation is in- 
stalled, the secondary voltage has to be kept constant by 
special regulation in any event. 

TRANSFORMERS. 

The alternating current transformer is merely a glorification, 
as it were, of the fundamental idea shown in Fig. 4, page 12. 
The loops A and B are expanded into massive coils and are 
given a very perfect magnetic core of laminated iron, but the 
principle is unchanged. 



TRANSMISSION BY ALTERNATING CURRENTS. 199 

In Fig. 103, ^ is a core composed of soft iron plates perhaps 
x^^ inch thick, stamped into the form shown, and then built 
up together like the leaves of a book, 5 is a coil of insulated 
wire wound in a spiral around one side of the core, and C is a 
single loop of heavy insulated copper bar around the other side. 
Now suppose an E. M. F. is suddenly applied to the terminals 
of the coil B, the loop C being left open. Current will flow 
through B in amount determined by its resistance and induc- 
tance, setting up a magnetic field throughout the mass of A. 
If the current is an alternating one an alternating magnetic 
field will be set up in A, and the current in B will settle down 
to that value which is determined by the resistance and induc- 




FlG. 103. 



tance of the coil. The energy represented by this current is 
spent in heating the coil and in doing work by the reversal of 
magnetism in the core A. The current thus engaged lags 
behind its E. M. F. as in other cases of inductive circuit, the 
power factor at no load being in ordinary cases from .6 to .7. 
Now close the loop C. Current opposing the current in B 
will be at once set up. The magnetizing effect of this reverse 
current opposes the magnetization due to B, and hence tends 
to cut down the inductance imposed on B, which is^ as we have 
already seen, determined by the magnetic induction through 
its core. To this action B simultaneously responds with an 
increased current, so that any increase of the current in C and 
its consequent demagnetizing action, is automatically compen- 



200 ELECTRIC TRANSMISSION OF POWER. 

sated by an increased current in B. The increase of energy 
represented by this compensates for the energy due to the 
current in C. Energy is thus virtually transferred from the 
primary circuit B to the secondary circuit C. 

Now as to the voltage of these two circuits. The energy 
in the two circuits is evidently equal save for losses in the iron 
and copper, which amount ordinarily to only a few per cent. 

For any given magnetization in A the inductive E. M. F. 
in B is proportional to the total number of turns in the coils ; 
so also the induced E. M. F. in the secondary is proportional 
to the number of turns in it. That is for a certain rate of 
change of the magnetic induction in A, the induced E. M. F. 
is the same per turn throughout A, whether that E. M. F. 
appears as inductance in B or secondary E. M. F. in C. 
Hence, the E. M. F.'s across the terminals of the primary and 
secondary coils are proportional to the respective numbers 
of turns in those coils. But the energy in the two is sub- 
stantially equal, and hence the currents in primary and secon- 
dary must be inversely proportional to the respective E. M. F.'s 
In Fig. 103 are shown seven primary turns and one secondary 
turn. Therefore, the secondary E. M. F. is one-seventh the 
primary E. M. F., and the primary current is one-seventh the 
secondary current. For- the same density of current in am- 
peres per square inch the secondary turn must have seven 
times the cross-section of the primary conductor. By simply 
changing the relative number of primary and secondary turns 
— the ratio of transformation — electrical energy at any vol- 
tage can be transformed to any other voltage with trifling loss 
if the apparatus be properly designed. 

The losses which exist are of three kinds. First is the loss 
due to the resistance of the copper. This at light loads is 
very trifling, but increases with the square of the load, being 
numerically equal in watts to C^ R, as in all cases of loss 
through resistance. 

Second comes the loss through hysteresis — virtually mag- 
netic friction — produced by the alternate reversals of mag- 
netization in the iron core. This is nearly constant at all 
loads and is kept as low as possible by securing the best pos- 
sible iron, and working it at rather low magnetization, since 



TRANSMISSION BY ALTERNATING CURRENTS. 201 

the hysteretic loss increases very rapidly as the iron is more and 
more strongly magnetized. 

Finally comes the loss from eddy currents in the core. 
This is due to the fact that the core is a fairly good conductor, 
and currents are induced in it for precisely the same reason 
that they are induced in the secondary winding. These eddy 
currents are largely reduced by carefully laminating the core 
across the natural direction of flow of these currents, and 
insulating the laminae with sheets of tissue paper or with 
varnish. The loss from eddy currents is, generally speaking, 
of about the same magnitude as the hysteretic loss, and in 
transformer practice the two are usually lumped together and 
denominated core loss. 

By careful construction and design these losses can be kept 
very small compared with the total output. The following 
data from a test of a 7,500 watt transformer designed for a 
frequency of 15,000 to 16,000 alternations per minute, about 
125 to 135^, will give a clear idea of the results that can be 
reached commercially even in small transformers. 

Output . 7.5 KW 

Transformation ratio 20 : 1 

Full load amperes (primary) 3.6 

Full load amperes (secondary) 72.0 

Resistance (primary) ohms 6.15 

Hesistance (secondary) ohms .012 

Total C2 R loss (watts) ........ 143. 

Total core loss (watts) 78. 

Primary current (no load) .063 

Power factor (no load) .595 

Total C R drop (per cent) . . 1.9 

The efficiency curve of this transformer at various loads is 
given in Fig. 104. The interesting feature of this curve is the 
very uniform efficiency from half load to full load, with a maxi- 
mum of 97.4 per cent at three-quarters load. This is the 
result of a relatively very small core loss. Even at one-tenth 
the normal load the efficiencj^ is still good, over 90 per cent, 
although the curve falls more rapidly below half load. 

The larger transformers, such as are used for heavy power 
transmission work, are even more efficient than the small one 



202 



ELECTRIC TRANSMISSION OF POWER. 



here described, although the room for increase is now very 
Hmited. Within the last few years the improvement in com- 
mercial transformers has been very great. In practice they 
are seldom so simple in form as in Fig. 103, the core plates 



100 



iu95 

h- 

UJ 

O 

en 

in 
a. 



90, 



/ 



3 4 5 

OUTPUT IN K.W. 

Fig. 104. 



being universally built up of several pieces, so that the coils 
may be wound in forms and slipped into their respective places 
on the core. One of the forms which has been widely used 
is shown removed from its case in Fig. 105. The hollow 
rectangle A forms the m.ain part of the core, while the bridge 
piece, B, is built up separately as the core of the coils, together ' 




B 



Fig. 105. 



with which it is forced into the position shown. The secon- 
dary coil immediately surrounds the bridge, and outside of it 
is the primary coil. Both coils are of course elaborately 
insulated. Another familiar form of transformer is shown in 



TRANSMISSION BY ALTERNATING CURRENTS. 203 

Figs. 106 and 107. Here the core is built up of straight rec- 
tangular slips of iron into a hollow rectangle upon the longer 
sides of which the coils are fitted, as in Fig. 106, separated 
by heavy sheet insulation in the manner shown. The whole 
assembled core and coils are shown in longitudinal section in 
Fig. 107. This form of construction gives the coils a large 
available cooling surface and simplifies their insulation some- 




FlG. 106. 



what, although magnetically the arrangement of Fig. 105 is 
to be preferred. 

As transformers are usually inclosed in tight iron boxes to 
protect them from the weather, the heat generated in the coils 
and core has a rather poor chance to escape, and the tempera- 
ture may therefore rise higher than is safe for the insulation. 
It is usual to take special precautions to prevent this over- 
heating. One of the commonest and best devices for this 




Fig. 107. ' 

purpose is the subdivision of the core into bunches of laminae 
separated by air spaces. 

This arrangement is well shown in Fig. 108, in which the 
core is provided with a dozen of these ventilating spaces. 
The arrangement of the coils is somewhat like that of Fig. 105. 
As an additional precaution against overheating, the trans- 



204 



ELECTRIC TRANSMISSION OF POWER. 



former case is often filled with heavy mineral oil after the 
core is in place. This both provides additional insulation, 
and facilitates the transfer of heat from the core and coils to 
the iron case, whence it is radiated to the surrounding air. In 
very large transformers the primary and secondary windings 
are often built up of thin flat sections assembled with spaces 
between them. 

For huge transformers such as are used for substation 
work, means are geaerally provided for artificial cooling. Two 
methods are at present in use for this purpose. One is the 
use of a blast of air from a small blower streaming through 




Fig. 108. 

the interstices provided in core and coils, and rapidly carrying 
away the heat generated. The other is applied to oil-filled 
transformers, and consists in cooling the oil by a worm in 
the transformer case through which cold water is allowed 
to flow, or with a small pump circulating the oil itself slowly 
through a worm cooled by water. Either plan is very effective, 
and both are extensively used. 

With properly designed tranformers there is no difficulty in 
dealing with any voltage now in use, without the device of 
connecting transformers in series, which was formerly often 
employed for high voltage. Plate VI shows the latest trans- 
former practice, the interior of a water-cooled 950 KW West- 
inghouse transformer. It is designed for use at 25 <^ to give 
50,000 volts upon the transmission lines. Its splendid efficiency 









«*Sj«g«:'f,»^- ■ 








I 




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"4 

t 

1 

1 



PLATE VI. 



TRANSMISSION BY ALTERNATING CURRENTS. 205 

of a similar large transformer is shown in Fig. 109. If trans- 
formers are of similar size and design, they can be rim in 
parallel with the utmost facihty, and may very often be thus 
''banked" most advantageously, as with such connection it 
is easy to proportion the number of transformers in use to the 
load, so that they can be worked nearly at full load, and con- 
sequently at their best efficiency. 

In general the larger the transformer the higher its effi- 
ciency, though the improvement is very slow after the out- 
put reaches 25 KW or thereabouts. The curve of Fig. 110 



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91 


























































































































90 























































































































































































































































0.1 0.2 0.3 0.4 0.5 0,6 0.7 O.S 0.9 1.0 1.1 1.2 1.3 1.4 1.5 
PROPORTION OF FULL LOAD 

Fig. 109. 

shows the change in half -load efficiency with the size of trans- 
former as found in ordinary American practice. 

The data here given relate to transformers of the kind 
employed for power transmission work, as now produced by 
the best makers. The sizes above 50 KW are frequently 
artificially cooled. The frequency is taken at 60 ^ to 70 ^, 
and the figures do not apply to transformers originally 
designed for higher frequencies. At lower frequencies the 
efficiencies are likely to be a fraction of a per cent lower, but 
at any frequency within the range of ordinary working a first- 
class transformer of 50 KW capacity or upward can be 
depended on for a full load efficiency of just about 98 per 



206 



ELECTRIC TRANSMISSION OF POWER. 



cent, and a half load efficiency about one per cent lower. 
With care in planning a substation equipped with these large 
transformers, the loss under normal conditions of working 
should not exceed 2^ per cent. 

For polyphase work it is the almost universal custom in this 
country to employ simply groups of ordinary standard trans- 
formers. Abroad, composite transformers, transforming two 
or more phases in a single structure, are often used. The 
intent of this arrangement is to utilize more fully the iron 
core by making it common to the several phase windings. 
Three laminated cores, with the laminae running vertically, are 



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r 










































on 






















( 




5 





1( 


)0 


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sc 


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850 



K.W. OUTPUT 
Fig. 110. 



united at the ends by laminated yokes. Each core receives 
the primary and secondary windings belonging to a single 
phase, while the iron belongs to the three in common. The 
arrangement is akin to the mesh coimection of three-phase 
circuits. 

It is a question whether the common use of the core iron 
is a sufficient offset to the loss incurred in operative flexibility. 
Separate transformers for each phase can be readily shifted 
about or reconnected in case of accident, while if anything 
happens to a polyphase transformer it is likely to put out of 
action a considerably greater capacity than in the other case. 
Nevertheless, three-phase transformers are considerably used 
abroad and very recently they have come into current prac- 



TRANSMISSION BY ALTERNATING CURRENTS. 207 

tice here. Fig. Ill shows, removed from its oil-filled case, a 
three-phase transformer of about 50 KW capacity. The 
arrangement of the cores is akin to that of Fig. 107, but with 
three wound cores instead of two. Similar transformers are 
now being made of several thousand kilowatts capacity, but 
whether they will have a permanent place in the art remains 
to be seen. They are at present emphatically special, and it 
is somewhat dubious whether they present sufficient advan- 
tages to compensate for the extra capacity jeopardized in case 
of trouble. 




Fm. li 



Several arrangements of transformers are employed in poly- 
phase working corresponding to the various arrangements -of 
polyphase circuits. For example, in two-phase systems ' the 
transformers are generally connected as shown in Fig. 112. 
This is simply one transformer per phase connected in the ordi- 
nary manner. The tv/o phases are kept distinct both as regards 
primary and secondary sides of the circuit. Fig. 113 shows 
the composite circuit method of connection. Both primary 
and secondary circuits have one wire common to both phases. 
In this case there is between the outside wires of the System 
a higher voltage than exists between either outside wire arid 
the common wire. This voltage is of course the - geomet-rical 



208 



ELECTRIC TRANSMISSION OF POWER. 



sum of two separate phase- volt ages. As these are 90° 
apart the resultant voltage is V2 times either component. 
Not infrequently the primary arrangement of Fig. 112 is com- 
bined with the secondary circuit of Fig. 113. This is the ordi- 
nary connection of two-phase motors, which are often built for 
this three-wire circuit. As a rule all lighting connections and 
all long circuits of any kind are made as shown in Fig. 112. 

Transformers for three-phase circuits, are, like the circuits 
themselves, very seldom worked with the phases separated, 
but in nearly every case are combined in the star or mesh 




Fig. 112. 



connection. The former is useful in dealing with very high 
voltages, since the individual transformers do not hat^e to carry 
the full voltage between lines. Fig. 114 shows a diagram of 
the star connection and Fig. 115 the corresponding mesh. In 
each a, b, c, are the primar}^ leads, and A, B, C the correspond- 
ing secondary leads. Of the two connections the mesh is 
rather the more in use except for high voltage work, and for 
secondary distribution with a connection to the common 
junction of the transformer system, which connection has for 
certain purposes very great advantages. 

Whether the star or the mesh connection is employed, one 



TRANSMISSION BY ALTERNATING CURRENTS. 209 



transformer per phase is required, and this condition is some- 
times inconvenient as rendering necessary the use of three 
small transformers where a two-phase system would need but 
two. To obviate this difficulty, what may be called the 



i'^ 



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II 



Fig. 113. 



"resultant mesh" connection is extensively used, particularly 
for motors. The principles on which this is based have already 
been set forth. 

Briefly, if one takes the geometrical sum of two E. M. F.'s 



6 (% 




differing in phase, the resultant will be less than the arithmeti- 
cal sum of the components, and not in phase with either. 
From the examples of geometrical summation already 
discussed, it is evident that by varying the magnitudes of the 
components and the angle between them, i.e., their phase 
difference, the resultant may have any desired value and any 
direction with reference to either component. 



210 



ELECTRIC TRANSMISSION OF POWER. 



The ''resultant mesh" three-phase connection is shown 
in Fig. 116. It is composed of two transformers instead of 
three as in Fig. 115, the E. M. F. between the points A and C 
being the resultant derived from the two existing secondaries. 
Each "of these secondaries contributes its part of the output 
in the resultant phase, and the secondary circuit behaves 
substantially as if it were derived from the ordinary mesh 
connection. This arrangement is very convenient in motor 
work, since it is very simple and allows the use of two trans- 
formers when desirable for the required output. Sometimes 
a motor is of a size that is fitted better by three standard 
transformers than by two, or the reverse, and with the choice 




Fig. 115. 



of the two mesh connections it is often possible to avoid some 
extra expense or to utilize transformers that are on hand. 

A very beautiful application of this principle of resultant 
E. M. F. is the change of a two-phase system into a three- 
phase, or vice versa. The method of doing this is shown in 
Fig. 117. Suppose we have two equal E. M. F.'s 90° apart, as 
in the ordinary two-phase system, as the primary circuit. The 
secondary E. M. F.'s will still be 90° apart, but can be of any 
magnitude we please. Let one of these secondaries A C give 
say loo volts, and tap it in the middle so that the halves, A D 
and D C will each be 50 volts; now wind the other secondary, 
B D, ioY 50 V3 volts, and connect one end of it to the middle 
point of the first secondary. Taking now the geometrical 



TRANSMISSION BY ALTERNATING CURRENTS. 211 

sums oi B D with the two halves of A C, the resultants are 
equal to each other and to A C, and leads connected to A, B, 
and C will give three equal E. M. F/"s 120° apart, forming a 
three-phase mesh with two resultant E. M. F.'s instead of one 




Fig. 116. 



as in Fig. 116. The actual connection of a 1,000 volt two-phase 
system to form a 100 volt three-phase secondary system is 
shown in Fig. 118 . Reversing the operation by supplying 
three-phase current to the three-phase side of the system 
gives a resultant two-phase circuit. 

This change-over process is valuable in that it allows a 

B 



A 



/ 



/ 



:Z. 



\ 



D 
Fig. 117. 



three-phase transmission circuit to be used for the saving in 
copper characteristic of it, in connection with two-phase 
generating and distributing plants, and permits two-phase and 
three-phase apparatus to be used interchangeably on the same 



212 



ELECTRIC TRANSMISSION OF POWER. 



circuit, which is sometimes advantageous. A somewhat 
analogous arrangement permits the transformation of a 
monocycUc primary circuit into a three-phase or two-phase 
secondary form, as may be convenient, and in fact any sys- 
tem with two or more phases may be transformed into any 
other similar system in the general manner described. 

It is worth noting that the three-phase-two-phase trans- 
formation shown in Fig. 118 can in an emergency be very 
readily made without special transformers if one has avail- 
able transformers of ratios 9: 1 and 10: 1, respectively, both 



1000 



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50 



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50 



rx 




100 



100— 



Fig. 118. 



these being obtainable commercially. For the latter tapped 
from the middle of the secondary, as is common for three- 
wire work, gives the left-hand half of Fig. 118, while the 9: 1 
transformer is sufficiently near the required ratio to give the 
rest of the combination. Such an extemporized arrangement 
is very serviceable in operating three-phase induction motors 
from two-phase mains or vice versa, and can be put together 
very easily. In default of this it is easy enough in using 
standard transformers of makes in which the secondary wind- 
ings are fairly accessible, to tap the secondary winding so as 
to leave about 12 per cent of it dead-ended, and this forms 
the supplementary transformer required. 

The electrician will do well to familiarize himself with the 
handling of transformers in all sorts of connections, for in a 



TRANSMISSION BY ALTERNATING CURRENTS. 213 

sudden emergency a little deftness in this respect will often 
extricate him from an uncomfortable corner. For instance, 
one can connect transformers backwards to get high voltage 
for testing; or with the usual three-wire secondaries trans- 
form twice and reach half the primary voltage, or put several 
secondaries in series, with the corresponding primaries in mul- 
tiple, or do many other things occasionally useful. The chief 
things to be borne in mind are that the normal currents in 
primaries and secondaries must not be exceeded, that the 
polarities must be kept straight and great care must be exer- 
cised not inadvertently to get any coils on short circuit. 



To Line- 



C=tijl 



p rq- 



\ 



urn: 



Fig. 119. 



One of the most useful temporary expedients is boosting the 
primary voltage by means of a standard transformer to meet 
excessive drop in a long feeder. The process is exceedingly 
simple, being merely the connection of the secondary in series 
with the line to be boosted, while the primary is put across the 
mains as usual. The result is that the feeder voltage is raised 
by nearly the amount of the secondary voltage. Fig. 119 
shows a convenient way of arranging the connections, in which 
one of the primary lines is so connected to a double throw 
single-pole switch that while boosting goes on with the switch 
in the position shown, on throwing the switch to the reverse 
position the booster is cut out and the line receives its current 
as usual. It must be remembered in such boosting that the 



214 ELECTRIC TRANSMISSION OF POWER. 

strain on the transformer insulation is more severe than usual, 
and in particular that the strain between secondary coils and 
core is the full primary voltage, for which provision is seldom 
made in insulating secondaries from cores. Hence, in rigging 
a booster transformer one of the oil-insulated type should be 
chosen, and it should be very carefully insulated from the 
ground. For the same reason the boosting transformer 
should be of ample capacity, so that it will not be likely to 
overheat, and should in general be treated rather gingerly, like 
any other piece of apparatus subject to unusual conditions. 
Nevertheless, it is capable of most effective service if properly 
operated. 

All these systems which involve resultant E. M. F.'s are 
open to certain practical objections which may or may not be 
important according to circumstances. 

In the first place, the resultant E. M. F. is less than the 
sum of the E. M. F.'s for which the transformers in the com- 
ponent circuits are wound. For instance, in Figs. 116 and 118, 
100 resultant volts are derived from transformers aggregating 
respectively 200 and 186.7 volts, through the secondaries of 
which the resultant current has to flow. In the former case 
one-third and in the latter case two-thirds of the total current 
is thus derived at a disadvantage, using up more transformer 
capacity for a given amount of energy than if the transformers 
were used in the normal manner. On a small scale the dis- 
advantage is seldom felt, but in heavy transmission work with 
large transformers it may be quite serious. 

Second, the disturbance of any one component voltage from 
drop or inductance, or any shifting of phase between the com- 
ponents from unequal lag, disturbs all the resiiltant E. M. F.'s 
This, again, may or may not be of importance, but it must 
always be borne in mind, as i^ every case of combined phases. 

It is possible by combinations of transformers similar to 
those described, to obtain at some sacrifice in transformer 
capacity a single-phase resultant E. M. F. from polyphase 
components, or to split up a single-phase current, by the aid of 
inductance and capacity, into polyphase currents. Neither 
process is employed much commercially, since both encounter 
in aggravated form the difficulties common to resultant phase 



TRANSMISSION BY ALTERNATING CURRENTS. 215 

working mentioned above, and others due to the special form 
of the combinations attempted. Combining polyphase cur- 
rents for a single-phase resultant is a process that would be 
very seldom useful, but the reverse process if it were success- 
fully carried out might be of very great importance in certain 
distribution problems, and especially in electric railway prac- 
tice, although in working on the large scale that offers the best 
field for alternating motors the disadvantage of two trolleys is 
at a minimum. One very ingenious method of splitting an al- 
ternating current into three-phase components is the following, 
due to Mr. C. S. Bradley, one of the pioneers in polyphase 



TMPLE SECTION or TRANSFORME 



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OF TRANSFORM! li 



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Fig. 120. 



work. His process is essentially twofold, first splitting the 
original current into a pair of components in quadrature 
and then combining these somewhat as in Fig. 118. The appa- 
ratus is shown in diagram in Fig. 120. Here A is the gen- 
erator, B the simple primary of one transformer element, D a 
condenser, n and I the sections of the compound transformer 
primary, and g, h, i, /, k the secondary transformer sections. 
The condenser D is so proportioned that acting in conjunc- 
tion with the compound primary n I the original current 
is split into two components in quadrature, in B and n I respec- 
tively. Then the secondaries are so interconnected as to 
produce three-phase resultant currents which are fed to the 



216 ELECTRIC TRANSMISSION OF POWER. 

motor M. The coil i gives one phase, the resultant of g and k 
another, and the resultant of h and j the third. The combi- 
nation of these resultants gives a more uniform and stable 
phase relation under varying loads than would be obtained 
from two-phase secondaries fed by B and n I respectively. 
The condenser is necessary in getting a correct two-phase re- 
lation in the primaries to start with, and even so the E. M. F.'s 
will not stay in quadrature under a varying load on the sec- 
ondaries unless the condenser capacity be varied, but the re- 
combination in the secondaries partially obviates this difficulty. 
A device brought out abroad by M. Korda for a similar purpose 
omits the condenser and splits the monophase current into two 
components 60° apart by variation of inductance alone, and 
these are utilized to give three-phase resultants. The phase 
relations thus obtained are, however, unstable, as must always 
be the case in phase splitting by inductance alone. For 
the energy supplied by a monophase current is essentially 
discontinuous, while the energy of a polyphase circuit has no 
periodic zero values, so that in passing from one to the other 
there should be storage of energy during part of each cycle such 
as is obtained by the condenser of Fig. 120. 



CHAPTER VI. 

ALTERNATING CURRENT MOTORS. 

The principles of the synchronous alternating motor are a 
snare for the unwary student of alternating current working, 
since they involve, when discussed in the usual way, rather 
complicated mathematical considerations. And the worst of 
it is that the generalized treatment of the subject often causes 
one to lose sight of the fundamental ideas that are at the root of 
alternating and continuous current motors alike. The sub- 
ject is at best not very simple, and unless we are prepared to 
attack the general theory with all its many considerations, it is 
desirable not to cut loose from the common basis of all motor 
work. 

Recurring to the rudimentary facts set forth in Chapter T, 
we see that an electric motor consists essentially of two work- 
ing parts — a magnetic field and a movable wire carrying an 
electric current. The motive power — torque — is due to the 
reaction between the magnetic stresses set up by the current 
and those due to the field. The refinements of motor design are 
concerned with the efficient production of these two sets of 
stresses and their coordination in such wise that their reac- 
tion shall produce a powerful torque in a uniform direction. 

In continuous current motors, for example, the field mag- 
nets are energized by a part or the whole of the working- 
current, and this current is passed, before entering the arma- 
ture, through a commutator like that of the generator, so 
that in the armature the direction of the currents through the 
working conductors shall be reversed at the proper time, so as 
to react in a uniform direction with field poles which are con- 
secutively of opposite polarity. Were it not for the commu- 
tator the armature would, on turning on the current, stick fast 
in one position, as may happen when there is a defect in the 
winding. 

Now, since the function of the commutator in the generator 

217 



218 ELECTRIC TRANSMISSION OF POWER. 

is to change a current normally alternating, so that it shall 
flow continuously in one direction, and since the object of the 
commutator in the motor is periodically to reverse this current 
in the armature coils, thus getting back to the original current 
again, one naturally asks the reason for going to all this trouble. 
Why not let the generator armature do the reversing instead 
of providing two commutators — the second to undo the work 
of the first? 

The reason is not far to seek. In a generator running at 
uniform speed the reversals of current take place at certain 
fixed times — whenever an arm.ature coil passes from pole to 
pole, quite irrespective of the needs of the motor. The com- 
mutator on the other hand reverses the current in the motor 
armature coils in certain fixed positions with respect to the 
field poles so as to produce a continuous pull, irrespective of 
what the generator is doing. 

If we abolish the commutators the motor will run properly 
only when the alternating impulses received from the gen- 
erator catch the armature coils systematically in the same 
positions in which reversal would be accomplished by the 
commutator. Hence for a fixed speed of the generator the im- 
pulses will be properly timed only when the motor armature 
is turning at such a speed that each coil passes its proper 
reversal point simultaneously with each reversal of the genera- 
tor current. If generator and motor have the same number 
of poles, this condition will be fulfilled only when they are 
running at exactly the same number of revolutions per minute. 
In any case they must run synchronously pole for pole, so that 
if the motor has twice as many poles as the generator, it will 
be in synchronism at half the speed in revolutions per minute, 
and so on. 

If we try to dispense with the commutators when starting 
the motor from rest, the action will obviously be as follows: 
The first impulse from the generator might be in either direc- 
tion, according to the moment at which the switch was thrown. 
The reaction between this current in the armature coils and 
the field poles might tend to pull the armature in either direc- 
tion, but long before the torque could overcome the inertia 
of the armature a reverse impulse would come from the gen- 



ALTERNATING CURRENT MOTORS. 219 

erator and undo the work of the first. Consequently the motor 
would fail to start at all. 

If the impulses from the generator came very slowly indeed, 
so that the first could give the armature a start before the 
second came, the armature would stand a chance of getting 
somewhere near its proper reversal point before the arrival of 
the reverse current, and thus might get a helping pull that 
would improve matters at the next reversal, but the direction 
of the first impulse would be quite fortuitous. Starting the 
armature in either direction before the current is thrown on 
gives it a better chance to go ahead if the first impulses in the 
wrong direction are not strong enough to stop it altogether. 




TXUXJ 

Fig. 121. 

We see, then, that an alternating current derived directly 
from the generator does net give reversals in the motor coils 
that are equivalent to the action of a commutator, save at 
synchronous speed. Except at this speed the current from 
the generator does not reverse in the motor armature coils 
when the latter are in the proper position. 

Fig. 121 will give a clear idea of the conditions of affairs in 
the field and armature conductors of a continuous current 
motor. Here S and N are the poles, and + and — mark the 
positions of the positive and negative brushes with reference 
to the armature winding. The solid black conductors carry 
current flowing down into the plane of the paper. The white 
conductors carry current upward. The armature turns in 
the direction of the arrow, and as each conductor passes under 
the brush the current in it is reversed. This distribution of 



220 



ELECTRIC TRANSMISSION OF POWER. 



current is necessary to the proper operation of the motor, and 
if the brushes are moved the motor will run more and more 
weakly, and then stop and begin to run in the opposite direc- 
tion, until when the brushes have moved 180° the motor will 
be running at full power in the reverse direction. This final 
position mieans that the currents in the two halves of the 
armature have exchanged directions, so that the conductors 
originally attracted toward N and repelled from S, are now 
repelled from N and attracted toward S. If alternating 
current from the generator is led into the windings, the dis- 
tribution of current shown in Fig. 121 must be preserved, and 
since in abolishing the commutator the alternating current 
leads are permanently connected to two opposite armature 
coils through shp rings, the distribution of Fig. 121 can only 





Fig. 122. 



be preserved when these leads change places by making a half 
revolution every time the current reverses its direction. Other- 
wise the distribution of currents will be changed, and the 
motor will fail to operate, since each reversal of current will 
catch the armature in a wrong position, and may tend to 
turn it in the wrong direction as much as in the right. 

Hence such a motor must run in synchronism, or not at all, 
and to operate properly it must either be brought to full syn- 
chronous speed before the alternating current is turned on, or 
nursed into action by running the generator very slowly, work- 
ing the motor into synchronous nmning at very low speed, 
and then gradually speeding up the generator, thus slowly 
pulling the motor up to full speed. In practice the former 
method is uniformly employed, and the machine used as a 
synchronous motor is substantially a duplicate of the alternat- 
ing generator as already described. In fact, it is an alternating 



ALTERNATING CURRENT MOTORS. 221 

generator worked as a motor, just as a continuous current 
motor is the same thing as the corresponding generator. 

Fig. 122 gives a clear idea of the way in which synchronous 
alternating motors may be employed for power transmission. 
Here G is the generator driven from the pulley P. /S^ is a 
switch connecting the generator to the line wires L U . At the 
motor end of the line is a second switch S' , which can connect 
the line either with the synchronous motor M, or the starting 
motor M'. This latter is usually some form of self-starting 
alternating motor to which current is first applied. M' then 
gradually brings M up to synchronous speed ; when the switch 
S' is thrown over, the main current is turned on M, and then 
the load is thrown on the driving pulley F^ by a friction clutch 
or some similar device. 

Such a system has certain very interesting and valuable 
properties. We can perhaps best comprehend them by 
comparing them with the properties of continuous current 
motor systems. 

In the alternating system both generator and motor are 
usually separately excited, which means really that the field 
strengths are nearly constant; as constant in fact as those 
in a well designed shunt-wound generator and motor for con- 
tinuous current. 

Now we have seen that this latter system is beautifully self- 
regulating. Whatever the load on the motor, the speed is 
nearly constant, and the current is closely proportional to the 
load. If the load increases, the speed falls off just that minute 
amount necessary to lower the counter E. M. F. enough to 
let through sufficient current to handle the new load. The 
effective E. M. F. is the difference between E, the impressed 
E. M. F. and E' , the counter E. M. F. The current produced 
by this E. M. F. is determined by Ohm's law. 

E - E' 

C = (1) 

r 

where r is the armature resistance, and since we have seen that 
the output of the motor is measured by the counter E. M. F., 

W = CE' (2) 



222 



ELECTRIC TRANSMISSION OF POWER. 



where W, in watts, includes frictional and other work. E\ 
neglecting armature reaction, is proportional to the speed of 
the armature, which falls under load just enough to satisfy 
equation (2) by letting through the necessary current. 

Now we have seen that when we abandon the commutator 
the motor has to run at true synchronous speed, or else lose 
its grip entirely. How^ can it adjust itself to changing condi- 
tions of load? If the load increases, more current is demanded 
to keep up the output, but the field strength remains constant, 
and the counter E. M. F. of the motor cannot fall by reduction 
of speed. We must note that while in a continuous-current 
motor the counter E. M. F. of the armature is constant at 
uniform speed, in an alternating motor the counter E. M. F. 




Fig. 123. 



varies like that of the generator, following approximately a 
sinusoidal curve, as the position of the armature with respect 
to the field poles varies. 

Hence at any given instant the counter E. M. F., the speed 
and field strength remaining the same, depends on the position 
of the motor armature. In Fig. 123 we have a pair of alternat- 
ing machines, generator A and motor B. In normal running 
at light load, the two are nearly in opposite phase, since of 
course the impressed and counter E. M. F.'s are virtually in 
opposition. 

Now, if there is an increase of load the motor armature sags 
backward a little under the strain, thereby lessening the com- 
ponent of its counter E. M, F. that is in opposition to the 



ALTERNATING CURRENT MOTORS. 223 

impressed E. M. F. The current increases, and with it the 
torque, and the sagging process stops when the torque is great 
enough to carry the new load as synchronous speed. The 
change of phase in the counter E. M. F. thus takes the place 
of change of absolute speed in the continuous current motor, 
by the same general process of increasing the E. M. F. effec- 
tive in forcing current through the circuit. This effective 
E. M. F. is generally by no means in phase with the impressed 
E. M. F., and in general the current and the impressed E. M. F. 
are not in phase in a synchronous motor. Here, as elsewhere, 
the input of energy is 

C E cos 4>, 

while the output, which in the continuous current motor is 
simply the product of the current and the counter E. M. F., 
in the synchronous motor depends evidently on such parts of 
both as are in phase with each other, i. e., 

W = CE' cos <f>' (3), 

in which <t> is the angle between current and counter E. M. F. 
Likewise the current, which in the continuous current motor 
depends on the effective E. M. F. and the resistance, now de- 
pends on the counter E. M. F. and the impedance 1. So that 

E - E' 
C-— (4). 

In this equation the values of all the quantities depend on 
their relative directions, and by combining geometrically the 
factors of (4) we can form a clear idea of the singular relations 
that may be found in synchronous motor practice. 

The construction is similar to that found in Fig. 51, page 133. 

In Fig. 124, we will start with an assumed impressed E. M. F. 
of 1,000 volts, a counter E. M. F. of 800 volts and an impe- 
dance composed of 5 ohms resistance and 10 ohms equivalent 
inductance. 

To begin with, we will lay off the im.pressed E. M. F. A B, 
and then the coimter E. M. F. J5 (7, which as we have seen is 
in partial opposition to A B. In this case yl C is the resultant 
E. M. F., which, on the scale taken, is 300 volts. This, then, is 



224 ELECTRIC TRANSMISSION OF POWER. 

the available E. M. F. taken up by the inductive and ohmic 
drops in the armature. The next step is to find C (eq. 4) 
from /, and the value oi E — E' , just obtained. To obtain /, 
we must combine resistance and inductance, as shown in Fig. 55. 
Performing this operation, it appears that / = 11.18. Hence 

in the case in hand C = ^ 26.8 + amperes. As to the 

11.18 ^ 

direction of this current, we know that it is at right angles to the 

inductive E. M. F., i. e., is in phase with the resistance in Fig. 

125. Solving that triangle to obtain the angle between the 

current and impedance, it turns out to be a little over 63°, 

being the angle whose tangent is Laying off this angle a 

5 

from A C, the impedance in Fig. 1 24, we find the current to be 




Fig. 124. 

in the direction A D. This current then is out of phase with 

the impressed E. M. F. by the angle of lag D A^. It is also 

out of phase with the counter E. M. F., though by chance very 

shghtly, and lags behind the resultant E. M. F. A C, by the 

angle a. Being nearly in phase with the counter E. M. F., the 

gross output of the motor is approximately 26.8 x 800 = 21.4 

KW. 

Now, what happens when the load increases? The motor 

armature sags back a few degrees under the added torque, and 

the counter E. M. F. takes the new position B C\ The new 

resultant E. M. F. is A C, which on the scale taken equals 450 

450 

volts. The new value of the current is C = = 40.25 

11.18 

amperes, and its phase direction, 63° from A C, is A D\ The 



ALTERNATING CURRENT MOTORS. 



225 



new angle of lag is then D' A B, showing that under the larger 
load the power factor of the motor has improved. If C 5 
should lag still more, A C", together with the current, would 
keep on increasing. Evidently, too, the angle of lag D^ A B 
will grow less and less until A C^ B becomes a right angle, 
when in the case shown it will be very minute, and the power 
factor will be almost imity. Beyond this point the angle 
C^ A B will obviously begin to decrease, and D^ A B will begin 
to open out, again lowering the power factor at very heavy 
loads. 

Hence it appears that at a given excitation there is a par- 
ticular load for which the power factor is a maximum, and it 
is evident from the figure that in the example taken this maxi- 
mum will be higher as the inductance of the system decreases, 




1NDUCTANCE=10 
Fig. 125. 



and also will pertain to a smaller output. Let us now see 
what happens when the excitation of the motor is varied. In 
Fig. 126 the conditions are the same as before, except that we 
assume counter E. M. F.'s of 500 volts corresponding to C and 
1,100 volts corresponding to C\ Examining the former, the 
resultant E. M. Fo is ^ C = 528 volts, the corresponding cur- 
rent is 47 + amperes and the angle of lag D A B is much 
greater than before. The power factor evidently would still 
be rather bad under increased loads, and worse yet, when at 
lighter loads the angle ABC decreases. Lessened inductance, 
however, would help the power factor by decreasing the angle 
CAD, and hence BAD, Now, consider the result of in- 
creasing the motor excitation to B C = 1,100 volts. The 
resultant E. M. F. now becomes A C , being shifted forward 
nearly 90°, its value is 280 volts and the current is 25 + 



226 ELECTRIC TRANSMISSION OF POWER. 

amperes. But this current is now in the direction A D' , a 
being the same as before, and hence it no longer lags, but 
leads the impressed E. M. F. by nearly 45°. The power fac- 
tor is therefore still bad, but gets better instead of worse 
under loads greater than that shown. Inductance in the 
system now improves the power factor, and combined with 
heavy load might bring the current back into phase with the 
impressed E. M. F. 

The counter E. M. F.'s corresponding to C and C are 
rather extreme cases for the assumed conditions, but it is easy 
to find a value for the excitation which would annul the lag 
exactly for a particular value of the load. Laying off in Fig. 
126, C" A B ^ C A D' we find the required counter E. M. F., 
which is very nearly 910 volts. At the particular output cor- 



FlG. 126. 

responding to this condition, the power factor is unity, the 
current and the impressed E. M. F. are in phase, and since 
the current is therefore a minimum for the output in question, 
the efficiency of the conducting system is a maximum. At 
this point, too, the energy is correctly measured by the product 
of volts and amperes, so that if wattm.eters are not at hand 
the input at a synchronous motor can be closely approximated 
at any steady load by varying the field until the armature cur- 
rent is a minimum, and reading volts and amperes. 

Throughout this investigation it has been assumed that the 
ratio of resistance and inductance has been constant. This is 
not accurately true, but is approximately so when the induc- 
tance is fairly low. The phenomenon of leading current in a 
synchronous motor system does not indicate that the current, 



ALTERNATING CURRENT MOTORS. 227 

in some mysterious way, has been forced ahead of the E. M. F. 
which produces it, for the impressed E. M. F. is not responsible 
for the current, which is determined solely by the resultant 
E. M. F. behind which the current invariably lags. 

The net practical result of all this is that a snychronous 
alternating motor, under varying excitation, is capable of 
increasing, diminishing, or annulling the inductance of the sys- 
tem with which it is connected, or can even produce the same 
result as a condenser in causing the current to lead the im- 
pressed E. M. F. The maximum torque of the motor, which 
determines the maximum output, is determined by the greatest 
possible value of C E' cos <f>' consistent with the given im- 
pedance and electromotive forces. The stronger the motor 
field, and the less the armature inductances and reactions of 
both generator and motor, the greater the ultimate load that 
can be reached without overburdening the motor and pulling 
it out of step. 

As regards the relation in phase between current and im- 
pressed E. M. F., the three commonest cases are those for 
which the currents were computed for Figs. 125 and 126. The 
first, and commonly the most desirable, is that in which the 
current lags slightly at small loads, gradually lags less and less, 
comes into phase, or very nearly so, at about average load, and 
lags slightly again at heavy loads. The maximum efficiency 
of transmission, reached when the lag touches zero, is then at 
about average load. The second and commoner case is when 
the motor is rather under-excited, so that the lag merely 
reaches a rather large minimum, never touching zero. The 
third case is that in which the current leads at all moderate 
loads, passes through zero lag, and then lags more and more. 
The average power factor may be the same as in the first case, 
but more energy is required for excitation, and no advantage 
is gained except in carrying extreme loads, often undesirable on 
account of overheating, or in modifying the general lag factor. 

It is highly desirable for economy in transmission that the 
product of current and E. M. F. should be a minimum for the 
required load. This condition can be fulfilled for the motor 
circuit at any load by changing the excitation until the current 
for that load becomes a minimum. Further, the field of a 



228 



ELECTRIC TRANSMISSION OF POWER. 



uniformly loaded motor may in the same way be made to bring 
the entire line current of the system to a minimmn if the 
motor be of sufficient capacity. Thus a synchronous motor 
load can be made very useful in improving the general condi- 
tions of transmission. By changing the motor excitation as 
the load on the motor of the system varies, the power factor 
can be kept at or near unity for all working loads. 

Fig. 127 shows the power factor of a synchronous motor 
somewhat under-excited, and that of a similar machine with a 
field strong enough to produce lead at moderate loads. With 



















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PROPORTION OF FULL LOAD 
Fig. 127. 



proper adjustment of its field, the effect of a synchronous 
motor on the general conditions of distribution is very bene- 
ficial. In curve A, Fig. 127, the indications are that the motor 
had rather a high inductance and armature reaction, and the 
excitation was decidedly too low for good results. Curve B 
is from a 300 HP motor, with its field adjusted for zero lag at 
about I load. The inductance was low and the armature 
reaction small. The result is somewhat startling. Even at i 
load the power factor (current leading) is about .93. At half 
load it has passed .99, touches unity, and then slowly diminishes 



ALTERNATING CURRENT MOTORS. 



229 



to very nearly .98 (lagging) at full load. In this case the gen- 
erator was held accurately at voltage while the excitation of 
the motor was uniform. Both were polyphase machines wound 
for 2,500 volts. 

When a synchronous motor is used in this manner, it obvi- 
ously will show, at the same load, values of the current varying 
if the excitation be varied. For any load the minimum current 
is given by that excitation which brings the current into phase 















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10 20 

AMPERES IN FIELD 
Fig. 128. 



with the impressed E. M. F. This point is fairly well defined. 
At less excitation the current lags, with more it leads. 

Fig. 128 shows for a particular instance the relations between 
the current and the excitation of the motor field, at full load 
of the motor. It is evidently easy to adjust the excitation to 
the proper point. 

In the practical work of power transmission the synchronous 
motor has several salient advantages to commend it. At con- 
stant frequency it holds its speed absolutely, entirely indepen- 
dent of both load and voltage until, from excessive load or 
greatly diminished voltage, it falls out of phase and stops. 



230 ELECTRIC TRANSMISSION OF POWER. 

It constitutes a load that is substantially non-inductive, so 
that it causes no embarrassing inductive complications in the 
system, and takes current almost exactly in proportion to its 
work. 

Finally it can be made to serve the same end as a condenser 
of gigantic capacity in compensating for inductances else- 
where in the system, and thus raising the general power factor 
substantially to unity. 

As compensating disadvantages, it must run at one fixed 
uniform speed under all conditions, it is not self -starting, and 
it requires the constant use of a continuous-current exciter. 

For many purposes the fixed speed is no objection, and in 
most large work the exciter can be used without inconvenience. 
Inability to start unaided, even when quite unloaded, is on the 
other hand a very serious matter, and has driven engineers to 
many ingenious subterfuges. The simplest of these is to pro- 
vide a starting motor, which is supplied with power by any 
convenient means, and serves to bring the main machine up to 
synchronous speed. Then the main current is thrown on, the 
motor falls into synchronism, and the load is taken up by 
means of a clutch. The difficulty is to start the starting 
motor. In transmissions of moderate length, continuous 
current may be delivered over the main line from the exciter 
of the generator to the exciter of the motor, which is there- 
by driven as a motor, and brings the alternating motor up 
to speed. As the energy required for this w^ork is not great, 
say 10 per cent of the whole power transmitted, it can often 
be delivered quite easily. At long distances, however, the drop 
becomes too great for the moderate voltages available with 
continuous current, and other methods have to be used. 

The best known of these is that indicated in Fig. 122 in 
which the synchronous motor is brought up to speed by an 
induction motor and then clutched to its load after which the 
induction motor is thrown out of action. 

Another method sometimes used is a special commutator to 
rectify the current applied to the main motor armature, thus 
directing the impulses so as to secure a small starting torque, 
enough to bring the motor to speed. Then the commutator 
is abandoned and the motor falls to running synchronously. 



ALTERNATING CURRENT MOTORS. 



231 



An ingenious modification of this plan is found in a type of 
self-starting synchronous motor built by the Fort Wayne 
Electric Corporation, shown in Fig. 129. 

This machine has a double-wound armature. The main 
winding is of the kind usual in alternators, wound in slots in 
the armature core, and the leads belonging to it connect with 
the collecting rings via the brushes on the pulley end of the 
shaft. 

The other winding is a common continuous current drum- 




FlG. 129. 



winding, laid uniformly on the exterior of the armature. 
It is provided with a regular commutator as shown in the 
figure. 

The field is of laminated iron, and the field coils are in dupli- 
cate, there being a coarse wire winding which in starting is in 
series with the comm^utated armature winding, and a fine wire 
winding cut out in starting, but used alone when the motor is 
at speed. 

The motor in question is started by turning the alternating 
current, reduced to a moderate voltage by transformation, into 



23^ ELECTRIC TRANSMISSION OF POWER. 

the series field and the commutated winding. The machine 
then starts with a good torque, and when it has reached syn- 
chronous speed, indicated by the pilot lamp on the top of the 
motor being thrown into circuit by a small centrifugal gov- 
ernor, a switch is thrown over, sending the main current 
through the alternating winding and closing the fine wire field 
circuit upon the commutator, at the same time cutting out the 
series coils. The motor then runs synchronously, the excita- 
tion being furnished by the fine wire winding. This construc- 
tion is best suited for rather small machines, as the double- 
winding is rather cumbersome for large motors. 

At present the tendency in synchronous motor practice is 
wholly toward the use of polyphase machines. These will 
start, when properly designed, as induction motors, or may be 
started by separate motors. When at speed the field excita- 
tion is thrown on, and the machine thereafter runs in synchro- 
nism. As such motors in starting, as induction motors, take 
a very heavy current they are generally provided with starting 
motors, although at a pinch they may be brought to speed 
at reduced voltage independently, especially if it is practi- 
cable to drop the frequency temporarily and thus to bring 
generator and motor up to speed together. Synchronous 
polyphase motors possess the same general properties as other 
synchronous motors, and as most power transmission work 
is now done by polyphase currents, they are widely used. 

In general transmission work, synchronous motors find 
their most useful place in rather heavy work, which can be 
readily done at constant speed. 

They have high power factors even when used for very 
varying loads, and are valuable in neutralizing inductance 
m the line and the rest of the load. Even when not deliber- 
ately used for this purpose, they raise the general power factor, 
and thus have a steadying effect that is very useful. When 
working under steady load and excited correctly, they almost 
eliminate the lagging current that sometimes becomes so 
great a nuisance in alternating current working. 

The polyphase synchronous motors will run steadily even 
if one of the leads be broken, working then as monophase 
machines, and by stiffening the excitation will generally carry 



ALTERNATING CURRENT MOTORS. 233 

their full normal loads without falling out of synchronism; but, 
of course, with increased heating. 

In one case that came to the author's notice, such an accident 
befell a three-phase synchronous motor, which went quietly on 
dri\dng its load of 1,700 looms for four hours, until the mill shut 
down at night. 

For small motor work synchronous machines are somewhat 
at a disadvantage, from the complication of the exciter and 
inability to start under load. In sizes below 100 HP they have 
been very generally superseded by the far simpler and more 
convenient induction motor, the use of which is a most charac- 
teristic feature of modern power transmission. In the use of 
synchronous motors, both monophase and polyphase, there has 
been often encountered an annoying and sometimes alarming 
phenomenon known as ''hunting," or where several machines 
are involved, as ''pumping. " In mild cases it appears merely as 
a small periodic variation or pulsation of the current taken by 
the motor, often sufficient to cause embarrassing periodic vari- 
ations in the voltage of the system. The frequency is ordi- 
narily one or two periods per second, varying irregularly in 
different cases, but being nearly constant for the same machine. 
The amplitude may vary from a few per cent of the normal 
current upwards. Generally the amplitude remains nearly 
constant after the phenomenon is fairly established, but some- 
times it sets in with great violence and the amplitude rapidly 
increases until the motor actually falls out of synchronism. 
This is usually the result of pumping between two or more 
motors, and seems to be especially serious in rotary convert- 
ers, not only throwing them out of synchronism, but throwing 
load off and on the generators with dangerous violence. 

Fig. 130 shows a facsimile of a record from a recording volt- 
meter showing the pulsation of the voltage on the system 
produced by the hunting of a 300-HP synchronous motor. It 
set in as the peak of the load came upon the system and per- 
sisted until the peak subsided, when it was gotten under con- 
trol, only to break out again when the late evening load fell off. 
During the early evening it was so severe as to produce pain- 
ful flickering in all the incandescents on the circuit. 

In this case the dynamo tender was inexperienced and had 



234 



ELECTRIC TRANSMISSION OF POWER. 



not acquired the knack of so juggling the field current as to 
suppress the hunting. A few months later the same system 
was in regular operation without the least trouble from hunt- 
ing, the operators by this time having been thoroughly broken 
in. In the majority of cases adroit variation of the field 
strength abolishes hunting, which almost always starts with a 




Fig. 130. 



sharp change in load or power factor. Just how to handle the 
excitation to obtain the best results is a matter of experiment 
in each particular case, but except in cases of unusually seri- 
ous character the knack is soon acquired. A rather strong 
field often steadies things, although if strong enough to pro- 
duce leading current the trouble is sometimes aggravated. 



ALTERNATING CURRENT MOTORS. 235 

But in this case, as in operating alternators in parallel, 
the best running conditions have to be learned by expe- 
rience. 

Much yet remains to be learned about the exact nature of 
hunting, but its general character is about as follows: A 
sudden change in current or phase causes the armature to seek 
a new position of equilibrium. In so doing the sudden change 
in the armature reaction mom.entarily changes the field 
strength, which aggravates the instabilty already existing and 
causes the armature under the influence of its own inertia to 
overreach and run beyond its normal position of equilibrium. 
Then the field recovers and the armature swings back, once 
more shifting the field and again overrunning, and so on ad 
nauseam.. The pulsation of the exciting current in cases of 
hunting is generally very conspicuous, and the periodicity of 
the himting seems to correspond in general with the time con- 
stant of the field magnetization. 

A fly-wheel on the motor or direct connection to a heavy 
machine generally increases the trouble by increasing the 
mechanical momentum of the armature, while belted and flex- 
ibly connected motors suffer less. Heavy drop in the supply 
lines, which makes the voltage at the motor sensitive to varia- 
tions of current, and low reactance in the armature, which 
favors large fluctuations of current, are conditions specially 
favorable to violent hunting. Rotary converters in which the 
armature current and its reactions are very heavy, compared 
with that component of the current which is directly concerned 
with the rotation of the machine as a synchronous motor, are 
subject to peculiarly vicious hunting, which has often risen to 
the point where it threw the rotary out of synchronism. 
They are far less stable in this particular than ordinary syn- 
chronous motors, and cannot readily be controlled by varying 
the excitation on account of the consequent variation of vol- 
tage on the continuous current side. 

Motors and rotaries having their pole pieces not laminated, 
but solid, often show less tendency to hunt than machines with 
laminated poles. If the poles are solid any violent swaying of 
the armature current with reference to them is checked and 
damped by the resulting eddy currents, so that the hunting is 



236 



ELECTRIC TRANSMISSION OF POWER. 



pretty effectively choked. For the same reason alternators in 
parallel are less likely to pump if they have solid poles, and most 
foreign machines are built in such wise. Here, where laminated 
poles are just now the rule, recourse is had to "bridges" or 
"shields." These are essentially heavy flanges of copper or 
bronze attached to the edges of the poles, so that fluctuations of 
armature reaction and of field are damped by heavy eddy cur- 
rents whenever they arise, the bridges acting indeed like a 
rudimentary induction motor winding. An example of such 
practice is shown in Fig. 131, which shows a portion of the 




Fig. 131. 



revolving field of a large polypnase machine fitted with mas- 
sive castings, bridging the spaces from pole piece to pole piece 
and serving at once to hold the field coils rigidly wedged into 
place and to check pumping. A similar device is used in con- 
nection with many rotary converters with a very fair degree 
of success. Occasionally pumping may be traced to some 
definite cause like a defective engine governor having a periodic 
vibration, but more often the phenomenon is purely electro- 
magnetic. The use of shields or solid pole pieces constitutes 
the best general remedy, for, while adjustment of the field 
is often effective, it is often desirable to adjust the field 



ALTERNATING CURRENT MOTORS. 237 

for other purposes, and the necessitj^ of varying it to 
suppress hunting is sometimes very embarrassing, if not 
impossible.* 

INDUCTION MOTORS. 

An induction motor is a motor into which working current 
is introduced by electromagnetic induction instead of by 
brushes. It has therefore two distinct, although coordinated, 
fmictions — transformer and motor. To understand its action 
Ave must take care not to confuse these functions, and this is 
best done by recurring to the fundamental principles that are 
at the root of all motors of whatever kind. 

An electric motor consists of these essential parts, viz.: A 
magnetic field, a movable system of wires carrying electric 




Fig. 132. 

currents, and means for organizing these two elements so as 
to produce continuous torque. 

These parts are beautifully shown in their elementary 
simpHcity in Barlow's wheel, Fig. 132, invented some three- 
quarters of a century ago. 

In this machine A^ S is the permanent field-magnet, the 
arms of the star-shaped wheel are the current-carrying con- 
ductors, and a little trough placed between the magnet poles, 
and partly filled with mercury, serves with the wheel as a com- 
mutator. Its function is to shift the current from one con- 
ductor to the next following one, Avhen the first passes out of 
an advantageous position. In other words it keeps the cur- 
rent flowing so as to produce a continued torque, irrespective 
of the movement of the conductors. Such is precisely the func- 

* For the mathematical theory of the subject see Steinmetz, Trans. 
A. I. E. E. May, 1902. 



238 



ELECTRIC TRANSMISSION OF POWER. 



tion of the modern commutator, and it is interesting to note 
that the device of making the armature conductors themselves 
serve as the commutator is successfully used in some of the 
best modern machines. 

These same fundamental parts are found alike in motors 
designed for continuous or for alternating currents. We have 
already seen that a series-wound motor can serve for use with 
both kinds of current, since the commutator distributes the 
current alike for both, and since the direction of the torque is 
determined by the relative direction of the main field and that 
due to the moving conductors, alternations which affect both 
symmetrically leave the torque unchanged. 




Fig. 133. 



We have seen also that if the distribution of currents given 
by the commutator can be simulated by supplying the arma- 
ture with alternating impulses timed as the commutator would 
time them, we can dispense with the commutator, and sub- 
stitute two shp rings. In this case, however, the motor will 
run only when in synchronism, since then only will the alternat- 
ing impulses from the generator be properly distributed in the 
armature, as has already been explained. Besides, the current 
has to be introduced into the armature through brushes bear- 
ing on a pair of slip rings, and an exciter is required to supply 
the field. If one could use an alternating field, and induce the 
currents in the armature as one would in the secondary of a 
common transformer, the machine would be of almost ideal 
simplicity. 



ALTERNATING CURRENT MOTORS. 



239 



This is what is accompHshed in the induction motor. The 
field is suppUed with alternating current, and the working cur- 
rent is induced directly in the armature conductors. 

To this end the brushes used in the previous examples may 
be replaced by a pair of inducing poles, carrying the primary 
windings, to which the armature windings play the role of 
secondary. Those armature windings are therefore closed 
on themselves, instead of being brought out to slip rings. 

For this short-circuited winding various forms are employed, 
the simplest being shown in Fig. 133. It consists of a set of 
copper bars thrust through holes near the periphery of the 




Fig. 134. 



laminated armature core, and all connected together at each 
end by heavy copper rings. 

The simplest arrangement of field and inducing poles is 
showTi in Fig. 134. Here each pair of opposite poles is provided 
with a separate winding, so that the circuit A A supplies alter- 
nating current to one pair and B B to the other pair. The 
armature we will assume to be like Fig. 133. Now apply an 
alternating current to A A. The windings of the armature 
which enclose the varying electromagnetic stress will have set 
up in them a powerful alternating current almost 180° behind 
the primary current, i.e., in general opposed to it in direction, as 
considerations of energy require. The armature will not turn, 



240 ELECTRIC TRANSMISSION OF POWER. 

however, for two very good reasons: first, the current in it is 
far out of phase with the magnetization of the poles; and 
second, this current is quite symmetrical with respect to 
the poles, so that the only effect could be a straight push or 
pull without the slighest tendency to attract or repel one 
side of the armature more than the other. 

To produce rotation as a motor, there must be not only a 
current in the armature conductors, but there must be field 
poles magnetized and disposed so as to produce a torque upon 
these conductors. 

Suppose, now, an alternating current to be sent around the 
circuit B B. If it is applied simultaneously with the current 
in A A, we shall be no better off than before, for since the two 
pairs of poles act together and just alike, there is no magnetiza- 
tion in phase with the armature current, and nothing to cause 
the armature to turn either way. 

To obtain rotation we must arrange the two sets of poles so 
that one pair may furnish a magnetic field with which the cur- 
rent induced by the other pair is able to react. The simplest 
way of doing this is to supply B B with current 90° in phase 
behind the current in A A. Then when the current induced 
by A A rises, it finds the poles B B energized and ready to 
attract it, for the magnetization in B B and the current are 
less than 90° apart in phase. The less the lag of the arma- 
ture current behind its E. M. F., the more nearly will the 
magnetization of these field poles be in phase with the armature 
current, and the more powerful will be the torque produced. 
The B B set of poles necessarily induce secondary currents in 
the armature in their turn, toward which the A A poles serve 
as field during the next alternation. The directions of both 
armature current and field magnetization are now reversed, 
so that, as in the commutating motor, the torque is unchanged. 
The next alternation begins the cycle over again, and so the 
motor runs up to speed. Its direction of rotation depends 
evidently upon the relative directions of magnetization in the 
two sets of poles, for these determine the direction of the 
armature current and the nature of the field poles that act 
upon it. Reversing the current in A A or B B will therefore 
reverse the motor, while reversing both will not. 



ALTERNATING CURRENT MOTORS. 241 

The speed of the armature is determined in a rather inter- 
esting manner. When the armature is in rotation the electro- 
magnetic stresses which act upon a given set of armature 
conductors are subject to variation from two causes. First is 
the variation in magnetization, due to changes in the primary 
current; second, the variation due to the armature coils mov- 
ing as the armature turns, so as to include more or less of the 
magnetic stress. The E. M. F. in the armature conductors is 
due to the summed effect of these two variations. And since 
the two are in opposition, if the armature were moving fast 
enough to make a half revolution for each alternation of the 
field, the E. M. F. produced would be zero, since the rates of 
change in the field and in the area of stress included by the 
armature coils would be equal. 

This means that the armature must always run at less than 
synchronous speed — enough less to produce a net armature 
E. M. F. high enough to give sufficient armature current for 
the torque needed. 

Under varying loads, therefore, an induction motor behaves 
much Hke a shunt-wound continuous current motor. In both, 
the armature current is due to the net effect of an applied and 
a counter E. M. F., the former being delivered from the line 
through brushes in the one case and by induction in the 
other. In neither case can the speed rise high enough to 
equalize these two E. M. F.'s. There is, however, a very curi- 
ous and interesting forra of induction motor which runs at 
true synchronous speed until the load upon it reaches a certain 
point, when it falls out of step like any other synchronous 
motor, or under certain circumstances falls out of synchronism 
and then operates like an ordinary asynchronous motor. 

Its operation in synchronism seems a paradox at the first 
glance; but the principle involved is really simple, although 
the exact theory of the motor is a bit complicated. As has 
already been noted, if the rates of change of magnetic induc- 
tion due to the pulsation of the field and to the cutting of 
the field by the armature coils are equal and opposite, there 
will be no E. M. F. in these coils, and obviously no energy 
can be transferred from field to armature. If, however, the 
E. M. F. wave due to the change of magnetization in the field 



242 ELECTRIC TRANSMISSION OF POWER. 

and that due to the motion of the armature coils through the 
field are very different in shape, there can still be a periodical 
resultant E. M. F., generally of a very complicated description, 
accompanied by a transfer of energy even at full synchronous 
speed. A very irregular wave shape in the E. M. F. of supply, 
or a distortion of it due to extraordinary armature reactions, 
may produce this condition. Fig. 135 shows the primary 
E. M. F. wave form as taken by the oscillograph across the 
terminals of such a synchronous induction motor, and the cor- 
responding current wave, which emphasizes the significance of 
the facts just given. The condition is best reached in small 
motors having sharply sahent field poles. The writer has 
never seen one which would start from rest imaided, the great 




Tig. 135. 

field distortion necessary being in the way, but once spun up 
to or near synchronism they work admirably on a small scale. 
The conditions of energy supply are obviously such as to be 
highly unfavorable in motors of any size, but for laboratory or 
other purposes where synchronous speed is wanted they are 
very convenient for an output of J HP or so, and form a 
very striking modification of the ordinary induction motor. 
They have, up to the present, been made mostly by the Holt- 
zer-Cabot Electric Co. 

If the load on an induction motor increases, demanding an 
increased torque, the armature slows down a trifle, until the 
new armature E. M. F. and resulting current are just sufficient 
to meet the new conditions. In the continuous current motor 
this speed is determined by the resistance of the armature, to 
which the current corresponding to a given decrease of speed 



ALTERNATING CURRENT MOTORS. 



243 



is necessarily proportional. In the induction motor the arma- 
ture resistance plays a precisely similar role. Fig. 136 shows 
the actual speed variation of a 100 HP induction motor in terms 
of its output. The maximum fall in speed under full load is a 
trifle less than 3 per cent, and even this result is sometimes 
surpassed in induction motors for especial purposes, even a 1 
per cent variation having been reached. A motor with higher 
armature resistance would fall more in speed, like a shunt 
motor with a rather high armature resistance. We thus see 
that the induction motor, as it should, behaves much like any 
other motor; the torque is produced in the same way, and 
obeys simdlar laws; the motor is similarly self -starting, and 
works on the same general principles throughout. Obviously 
the magnitude of the armature current in an induction motor is 



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Fig. 136. 



determined, not by the armature resistance alone, but by its 
impedance. As, however, the presence of reactance shifts the 
phase of the current, and that component of current which is 
effective in producing torque depends upon the resistance, 
the relation just explained holds good. That current is deliv- 
ered to the armature by induction is a striking feature, but 
not one that implies any radical difference in principle. 

It is not even necessary to use a polyphase circuit for work- 
ing induction motors, for, under certain conditions, the same 
set of poles can perform the double duty of delivering current 
and interacting ^vith it to produce torque. 

The principles of the induction motor, as here given, thus 
become part of the general theory of the electric motor which 
applies alike to machines for continuous and alternating cur- 
rent, quite independent of particular methods of construction 
or operation. 



244 



ELECTRIC TRANSMISSION OF POWER. 



The great pioneers in induction motor work, Tesla, Ferraris, 
and some others, preferred to view the matter from the special 
rather than the general standpoint, and hold to the theor}^ of 
the rotary pole action of induction motors — very beautiful, 
mathematically, but unfortunately hiding the kinship of induc- 
tion to other motors, and distracting attention from the trans- 
former action, which is so prominent. 

From this point of view the two pairs of poles in Fig. 134 




Fig. 137. 





Fig. 138. 



Fig. 139. 



co-act to produce an oblique resultant magnetization, which 
shifts around the field, producing a moving system of poles, 
following the sequence of the current phases, and dragging 
around the armature after them, by virtue of the currents in- 
duced in it. Figs. 137, 138, 139 show the rudimentary prin- 
ciples of the rotary pole. In Fig. 137 an annular field magnet 
is wound with two circuits A A and B B, supplied with alter- 
nating currents 90° apart in phase. The polarity of the 
armature is represented diagrammatically by the rotating 
magnet N S. 



ALTERNATING CURRENT MOTORS. 245 

Now, when the current in ^ ^ is maximum (and that in B B 
is consequently zero), the field has poles at P and P', which 
exert a torque on the armature poles. As the current falls in 
A A and rises in B B, the resultant poles move forward to P^ 
and P/ (Fig. 138), followed by the armature. When the cur- 
rent B Bis Si maximum, and A A has become zero, the poles are 
at P2 and P2' and so on. In order that the revolving poles 
may induce current in the armature, the latter must slip 
behind so as to produce relative motion and change in electro- 
magnetic stress. 

This point of view is very interesting and instructive. It 
deals, however, not directly with the two field magnetizations 
— the functions of which have just been discussed — but with 
a resultant rotary magnetic field, which may or may not have 
a concrete existence, according to circumstances. It by no 
means follows that because two equal energizing currents are 
90° apart in phase, they must or do form a resultant rotarj'- 
magnetic field, or that, if they are so organized as to give a 
physical resultant, their individual functions are superseded 
and must be neglected. 

The two views of the induction motor here set forth are not 
in any way conflicting; they inerely represent two methods of 
treatment of the same phenomena. As it happens, the rotary 
field point of view is from a mathematical standpoint the 
easier, for it treats the resultant instead of its components, 
and hence has been the oftener used, but in discussing certain 
classes of induction motors, it is by no means convenient, and 
is less general than the analytical method, which deals with the 
separate components. In most commercial induction motors 
there is undoubtedly a resultant rotary field, but however con- 
venient it may be to consider the motors in that light, it is not 
well to lose sight of the general actions of which the rotary 
field is a special case. 

As a matter of fact, the several currents in a polyphase in- 
duction motor may be so distributed that they cannot produce 
a resultant rotary magnetization, and in certain heterophase 
and monophase motors the ''rotary field," in so far as one is 
formed by the field, may revolve in one direction while the 
armature starts and runs strongly in the other direction. 



246 



ELECTRIC TRANSMISSION OF POWER. 



Hence, the view here taken of the induction motor has been 
generaUzed for the purpose of bringing out its relation to the 
general theory of motors, and to take account of induction 
motors, in explaining which the rotary pole theory would have 
to be, as it were, dragged in by the ears. 

Salient poles, hke those of Fig. 134, are seldom used, and 
the induction motor as generally constructed, consists of two 




-I11.1.I1.1 .,11. — i..>L....hi.ii, ,11 1| 




on 



Fig. 140. 



short concentric cylinders of laminated iron, slotted on their 
opposed faces to receive the windings. Sometimes these slots 
are open, and again they are simply holes close to the surface 
of the iron. 

The relation of the parts is well shown in Fig. 140, a 6 HP 
two-phase motor by C. E. L. Brown. 

In this case the exterior ring is the primary, and the revol- 
ving ring the secondary element of the motor. The primary 
winding is of coils of fine wire threaded through the core 
holes, while the secondary member is wound, if one may use 



ALTERNATING CURRENT MOTORS. 



247 



the term, with solid copper rods united at the ends by a broad 
copper ring. The clearance between primary and secondary 
is very small in all induction motors, almost always less than 
J inch, sometimes less than ^V inch. The smaller the clear- 
ance the better the machine as a transformer. 

The primary of an induction motor is wound much as the 
armature of a polyphase generator is wound, as described 
already. Fig. 141 shows in diagram a two-phase winding for a 
24 slot primary, and Fig. 142 a three-phase winding for the 
same primary. In the former there are two sets of coils, A 




Fig. 141. 



and B, each forming a separate phase winding; in the latter the 
bhree sets. A, B, C, may be united to form either a "star" or 
''mesh" three-phase winding. In practice the primary mnding 
is nearly always polydontal, for the same general reasons that 
hold for generator armatures, but especially to keep down 
inductance. For the sam_e reason the secondary winding is 
polydontal. As an example of the best usage in this respect, 
Fig. 145 shows the number and relation of primary and 
secondary slots in the motor shown in Fig. 140. There are no 
less than 40 primary slots for a four-pole winding, i.e., 5 slots 
per phase per pole, while the secondary has 37 slots, this odd 



248 



ELECTRIC TRANSMISSION OF POWER. 



number being chosen to reduce the variation in the magnetic 
relations of primary and secondary due to different positions 
of the armature. 

-Induction motors with fixed primary have the great advan- 
tage of having no moving contacts, and no high voltage wind- 
ings exposed to the strains due to revolution. On the other 
hand a revolving primary makes it very easy to vary the resist- 
ance in the secondary circuit, which is often desirable. Both 
forms are used, the latter only rarely. Inasmuch as a large 
proportion of the hysteretic loss occurs in the primary, since 




Fig. 142. 



in the secondary the variation of the magnetization is small, 
a revolving primary, being of less dimensions than its secon- 
dary, gives a slight advantage in efficiency. There is, however, 
small reason to suppose that on the whole it is easier to build 
one form than the other for a given efficiency with the same 
care in designing. In recent practice it is not uncommon to 
wind the primaries of large induction motors for voltages up 
to 10,000. 

Motors with revolving primary are no longer regularly man- 
ufactured, the vastly superior simplicity of the other con- 
struction being generally recognized. Plate VII shows the 





Fig. 2. 



PLATE VII. 



ALTERNATING CURRENT MOTORS. 



249 



motors now in common use in this country. Fig. 1 is a stan- 
dard Westing-house ''Type C C " motor of 10 HP. It is exceed- 
ingly simple in construction, and efficient in operation. It has 
a ''squirrel-cage'' armature similar to that of Fig. 133, but 
the bars are in open slots and are of rectangular section, a 
construction which gives a lower armature reactance than if 
the iron were closed over the armature bars. These motors 
start with a powerful torque, approximately two or three 
times the torque at rated full load, when the full line voltage 
is thrown upon the primary, but of course take, under these 
conditions, a very heavy current, so that in practice it is usual 
to start them at reduced voltage, which gives all the torque 



^ CIRCUCC 




CONNECTIONS FOR AUTO-STARTER 
I CIRCUIT 



riH 



ipn m r 



RUNNING fOSITKai 



OFP posrarow 

— ntlr-^ 



AUTO-STARTER INSTARTINCi P.OSITION 



i starting position 
Fig. 143. 




THREE PHASE 
MOTOR, 
^UTO-STARTER IN, RUNNING POSITION 



necessary without calling for excessive current. This is ac- 
complished by means of a so-called auto-converter, of which 
the essential connections are shown in Fig. 143. With the 
switch in the starting position the applied voltage is only a 
quarter or a half the normal voltage, the actual anioimt being 
adjusted by means of the variable connections shown, and 
when the motor has come up to its full speed under the starting 
conditions the switch is suddenly thrown over, putting the full 
working voltage in circuit. The actual appearance of the 
latest form of auto-starter is shown in Fig. 144. The starting 
voltage is applied in several steps and the whole device is 
immersed in oil. It is necessary to let the motor reach its full 



250 



ELECTRIC TRANSMISSION OF POWER. 



speed with the lower voltage before making this change, else 
there will be a needlessly severe current due to the sudden 
acceleration under full voltage, and the change should be 
made quickly, lest the armature speed should fall off during 
the change and produce the same unpleasant result. When 
intelligently handled the starting current can be kept within 
very reasonable limits, but the auto-converter should be ad- 
justed when set up to give at starting merely the voltage 
needed to start under the required torque, an excess of voltage 
meaning excess of current. Fig. 2 of Plate VII is a General 




Fig. L44. 



Electric ''Type L" motor of 35 HP. The mechanical design 
is very simple, giving a light and well ventilated structure. 
The bearings can be shifted to compensate for wear. The 
winding of the armature is a regular three-phase bar winding 
furnished with starting resistances within the spider, which are 
cut out gradually by means of a ring moved by the lever seen 
just within the bearing spider. The starting resistances are 
in many sections and can be short-circuited very gradually, 
holding the primary current practically constant from start to 
full speed, even when starting under a heavy torque. The 



ALTERNATING CURRENT MOTORS. 



251 



start is made with the lever pulled out to its fullest extent, 
and it is gradually pushed home until full speed is reached. 
Such motors are pecuHarly well adapted for use on lighting 
circuits, and in large sizes requiring heavy starting torque. 
The start can be made with very moderate currents, and the 
torque per ampere is considerably greater than in any motor 
starting on reduced primary voltage, which is the compensa- 
tion for the rather elaborate starting device. Neither of the 




Fig. 145. 



companies mentioned holds rigidly to the constructions here 
shown, but the cuts show their best standard practice. There 
is very little difference in the essential properties of the two 
forms, and both are very widely used. 

These recent motors are nearly all made with extremely 
small clearance between armature and field, from tV to ^V inch 
or less, even in large motors. This practice renders it easy to 
design for a good power factor, but may, and sometimes does, 
cause trouble mechanically, as might be anticipated. It is not 
difficult to make thoroughly good motors without resorting to 



252 



ELECTRIC TRANSMISSION OF POWER. 



such extreme measures, unless the designer is hampered by 
troublesome specifications in other particulars. Demand for 
slow speed motors at a periodicity of 60^, and insistence on a 
uniformity of speed at various loads that would not for a mo- 
ment be dem-anded in direct current motors, are responsible 
for serious and needless impediments in induction motor 
design. 

The ''Type L" motors just described have on the armature 




Fig. 146. 



a regular three-phase winding of rectangular bars united by 
end connectors. A simple four-pole form of such a winding is 
shown in Fig. 146. It obviously is more troublesome to con- 
struct than a "squirrel cage" winding, but it possesses certain 
advantages. Conspicuously, it renders it possible to insert the 
resistance in the secondary circuit at starting, which in the 
^'squirrel cage" would be a very difficult matter, although it 
has been tried. 



ALTERNATING CURRENT MOTORS. 



253 



If the field is very uniform, with a thoroughly distributed 
winding, there is very little difference in the actual perform- 
ance of the two kinds of armatures (drum-wound and ''squir- 
rel cage") when at speed. In case of a motor with salient 
poles or with few winding slots in the field, the drum armature 
has a very considerable advantage, owing to the fact that the 
currents in it are directed into definite paths which they must 
follow at all times, while in the ''squirrel cage" form the cur- 
rents are only uniformly organized when there is a uniform 
field. In the early motors, therefore, the drum-wound arma- 




FlG. 147. 

ture had a great advantage, but as the art of designing has 
advanced the two types have become closely approximated in 
their properties. 

In this country the windings of induction motors are gener- 
ally placed in open or nearly open slots, as in the case of the 
motors shown in Plate VII. Abroad the arrangement of wind- 
ings in holes as shown in Fig. 145 is very common. Each 
procedure has its advantages. The American practice renders 
it very easy to place the windings, and to put a very large 
amount of copper upon the armature, for open slots can be 
made radially deep and filled true, while holes unless rather 
large can only be trued by reaming, which implies a round 



254 



ELECTRIC TRANSMISSION OF POWER. 



hole, unfavorable if a great amount of copper is to be crowded 
upon the armature. Hence, with open slots it is easier to sub- 
divide the winding into many slots, thus reducing the armature 
reactance. On the other hand, open slots are extremely unfa- 
vorable as regards power factor, since the iron surfaces opposed 
in armature and field are very greatly reduced, and hence the 
tendency to use extremely small clearances in order to make 
the best of a bad matter. The European practice is on the 
whole better as regards power factor, but does not facilitate 
the construction of motors of very low armature resistance, 
and is considerably more difficult of proper execution. The 




Fig. 149. 



matter really hinges on the relative cost of labor here and 
abroad. With cheap labor the manufacturer can afford to go 
into little refinements if it is otherwise worth while, but at 
American labor-rates handwork has to be minimized. On the 
whole, the American motors are fully up to foreign standards 
in general design, although the tendency here has been to 
make a fetish of uniformity of speed, even at the expense of 
more important characteristics. 

In motors such as those just described, with distributed 
windings and no sharply defined polar areas, the consecutive 
exchange of motor and transformer functions among the wind- 
ings is almost lost sight of in the presence of the very apparent 
phenomenon of resultant revolving poles, but the appearance 



ALTERNATING CURRENT MOTORS. 



255 



of the latter is a necessary result of the persistence of the 
former. These induction motors are generally operated from 
the secondary circuits of transformers, although the large 
sizes (50 HP and upward) are sometimes wound for use of 
the full primary voltage up to 2,000 volts or more. 

An early form of induction motor which possesses some 
interesting features is the Stanley machine, shown in Figs. 147, 
148, 149. The field in Fig. 147 is composed of two separate 
rings of laminated iron, each having eight polar projections. 
These field rings are assembled side by side with the poles 
''staggered,'^ as shown in the cut. Each field is energized 
separately, one from each branch of a two-phase circuit. The 
armature. Fig. 148, is composed of two separate cores assem- 




FiG. 150. 



bled side by side. The secondary winding, Fig. 149, polyodon- 
tal as usual, is common to the two cores. The transformer 
and motor functions are here separated, for each half of the 
machine acts alternately as transformer and motor, each set of 
fields inducing current which serves for motor purposes in the 
other half of the machine. There is no rotary field in the 
ordinary sense of that term, since there is no physical resul- 
tant of the two field magnetizations, nothing but the alterna- 
tion of transformer and motor functions that is a characteristic 
of all polyphase induction motors. These motors, now sel- 
dom seen, have been generally used in connection with 
condensers to improve the power factor, and to facilitate this 
practice have been usually Avound for 500 volts. 

A step further in the direction of simplicity, but generally 
inferior to both polyphase and hetero phase forms, are the true 
monophase induction motors. The principle of these motors is 



256 



ELECTRIC TRANSMISSION OF POWER. 



shown in Fig. 150. Here there is but one set of poles energized 
by the circuit A, while h, c, d, are portions of the armature wind- 
ing, which may be a simple squirrel cage, or a complex bar 
winding similar to those used in polyphase motors. 

If A be supplied with an alternating current, induced cur- 
rents will be produced in the armature, out of phase with the 
field magnetization and symmetrical with respect to it, so that 
no torque is produced. 

If, however, we spin the armature up to nearly synchronous 
speed; the armature currents will lag, from self-induction, 




Fig. 151. 



behind the E. M. F. set up by the field, so that they have an 
angular displacement with respect to the field at a time when 
the latter is still active. There is, therefore, torque between 
these two elements — in the direction of the initial rotation. 
The motor will thus run, when once started, equally well in 
either direction. 

In every motor there must be not only a field magnetization 
and current in a movable conductor substantially in phase 
with each other, but there must be a stable angular displace- 
ment between the two in order to ensure continuous torque. 
In continuous current motors this displacement is secured by 



ALTERNATING CURRENT MOTORS. ' 257 

the position of the brushes. In polyphase induction motors it 
is obtained by the space relation of the sets of poles combined 
with the time relation of the two or more currents. 

In the monophase motor this angular displacement is due 
to the displacement of the armature currents by inductance. 
Hence, there is a particular value of the inductance correspond- 
ing to the best condition of torque, more or less than this 
being especially injurious in this type of motor. 

In practice monophase induction motors are built in very 




Fig. 152. 

much the same form as polyphase motors, and for the same 
reason, i.e., to make the structure good as a transformer. In 
fact, the same motor structures are often used for both types. 
Fig. 151 shows the manner of winding a six-pole monophase 
primary, homologous with Figs. 141, 142. A monophase 
induction motor of 120 HP by Brown, Boveri & Co., is shown 
in Fig. 152. Monophase induction motors are not yet used to 
any large extent in this coimtry, and abroad their use is gen- 
erally confined to motors much smaller than the example 
shown. 

A moment's reflection will show that while the supply of 



258 



ELECTRIC TRANSMISSION OF POWER. 



energy to a polyphase motor is substantially continuous, in 
monophase motors it is essentially intermittent, so that the 
latter give less output for the same structure, while the depen- 
dence of the torque on the armature inductance generally 
leads to low power factors. 

Nevertheless, cases arise in which it is extremely convenient 
to use single phase motors. There are still many small light- 




FiG. 153. 



ing plants equipped only with single pnase generators, the 
extreme simplicity of the circuits out-weighing the advan- 
tage of polyphase service in economy of copper and ready 
availability of motor service. For the occasional motors de- 
sirable on such systems some form of monophase machine is 
important. There are also some large lighting plants that 
serve their territory both by continuous current and in the 



ALTERNATING CURRENT MOTORS. 



269 



remoter districts by alternating current for which such motors 
are useful, and even on polyphase systems there are isolated 
demands for motors which can best be filled by a circuit run 
from a single phase. 

Monophase motors are, too, rather more cheaply installed 
on account of the simpler circuits and lower cost of trans- 
formers, so that there is a genuine though at present rather 
limited demand for them. As a result there have been deter- 
mined and measurably successful efforts to produce practical 
monophase motors capable of use at least in small sizes, with- 
out the impairment of general regulation likely to come from, 
low power factor and large current at starting. Abroad the 
ordinary monophase type just described is used, generally 

100 



90 



f^ 80 
t3 



S 70 



/ I 



10 
H. P. out put 



Fig. 154. 



started light and taking up its load with a clutch, but here a 
self-starting motor is universally demanded. 

One of the recent American contributions to the list of 
monophase motors is somewhat out of the ordinar}^ in that it 
starts as an induction motor by the aid of a commutator. 
This is the Wagner motor shown in Fig. 153. In its general 
construction it is a pure monophase motor with an armature 
winding the coils of which are at one end connected with a 
commutator. This has bearing on it a pair of brushes which 
close upon themselves those armature coils which are in such 
angular relation with the field magnetization as to give a 
strong motor reaction with it. By thus keeping in action 



260 



ELECTRIC TRANSMISSION OF POWER. 



only coils giving an efficient torque in one direction, the 
necessary directed torque at starting is secured, and when the 
motor reaches a predetermined speed a compact little centri- 
fugal governor throws over a short-circuiting ring, converting 
the motor into an ordinary monophase induction motor. It 
is possible to start under load with this device by drawing 
rather heavily on the mains for current, but in any except the 
smallest sizes it is better to start light. 

Fig. 155 shows the characteristic curves of a recent 4 HP 
Wagner motor, which gave a highly creditable performance for 
a monophase motor of so small size. It comes much nearer 



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Fig. 155. 



representing real commercial conditions than the curves of 
Fig. 154, and, as we shall presently see, does not make a bad 
showing as compared with the polyphase motors ordinarily 
found upon the market. 

Although monophase motors as a class start at a great dis- 
advantage compared with polyphase motors, they can be made 
to give pretty good results at load by extraordinary care in 
designing. Fig. 154 shows the curves obtained from a certain 
Brown motor by Professor Arno. The motor was nominally 
of 15 HP, but was evidently overrated at that load. Never- 
theless, within a certain range of load the performance of this 




Fig. 1. 




Fkj 2„ 



PLATE VIII. 



ALTERNATING CURRENT MOTORS. 261 

motor compares well with that of the best polyphase motors 
of similar size. This motor had an extremely small air gap, 
and shows doubtless a record performance in several respects, 
but it proves that barring the matter of starting it would 
be possible to turn out a pretty useful machine of the mono- 
phase type if anybody desired it, although it is certain that 
at anything like equal cost of construction the polyphase 
motor must retain the advantage. Fig. 1, Plate VIII, shows 
a recent monophase motor of Westinghouse make. It is in 
appearance hardly distinguishable from the polyphase motors, 
and its operative qualities are said to be excellent. Speed 
regulation, never any too easy in induction motors, is almost 
out of the question in the monophase form. Still, within its 
limitations it has its uses. 

A very ingenious flank movement was made by the General 
Electric Co., upon the monophase problem in a monophase 
induction motor with condensers. In polyphase motors the 
usefulness of condensers had been shown by the Stanley 
type previously mentioned, and the same device seems spe- 
cially useful in overcoming the low power factor to which 
the monophase form is especially prone. Plate VIII, Fig. 2, 
shows the 2 HP monophase motor of this type, mounted upon 
a base which contains the condenser. Its weight is 295 lbs., 
its nominal speed 1,800 r.p.m. at 60^ audits slip at full load 
2.75 per cent. Its full load efficiency is 75 per cent, and 
power factor 92 per cent. The condenser is hermetically sealed 
in a tin case and is connected not as a shunt to the whole 
field, but is closed upon an independent phase winding, so 
that the motor belongs rather to the split-phase class than 
to the strictly monophase. The armature likewise is given a 
winding akin to that of the ordinary polyphase motors and is 
provided with a starting resistance, in series with the armature 
windings at starting and cut out automatically as the arma- 
ture nears speed, by a centrifugal switch. An automatic 
clutch pulley is also provided on these motors to further facil- 
itate starting with moderate current. 

As might be expected from these features the motor is singu- 
larly free from starting and power factor difficulties, and, at 
the cost of some complication to be sure, meets the end for 
larly free from starting and power factor difficulties. The cost 
and complication of the condensers has seemingly proved com- 



262 



ELECTRIC TRANSMISSION OF POWER. 



mercially disadvantageous, so that this very interesting machine 
appears in use very seldom, and is apparently unlikely to retain 
a place in the art. 

Fig. 156 shows the characteristic curves from a 5 HP motor 
of this class. It will be observed that the full load power 
factor is .95 and the real efficiency at the same point .80, which 
is certainly an excellent showing. Obviously the power factor 
is a matter of proportioning the condensers, and in motors 
of this class of 10 HP, and upwards the power factor is raised to 
unity, or the current is even made to lead at certain loads. 

















































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! 4 5 

Horse Power 

Fig. 156. 



The various recent forms of monophase motor have come 
into somewhat considerable, though scattered use, mostly in 
sizes below 10 HP, although now and then motors of several 
times this power have been installed. When judiciously in- 
stalled they can undoubtedly be made to give good service. 

There has also very recently been introduced a most inter- 
esting type of single phase alternating current motor derived 
from and closely resembling in its properties the ordinary 
direct current series motor. It is in fact a series motor spe- 
cialized for alternating current working. 

The direction of rotation of a series motor depends entirely 



. ALTERNATING CURRENT MOTORS. 263 

oil the direction of the magnetizations produced by the field 
and armature respectively. Consequently it does not change 
if the direction of the current be reversed at the motor termi- 
nals, but onl}^ if it be reversed as between field and armature. 
Hence, such a motor would run even if supplied at its terminals 
with alternating current, provided enough such current could 
be forced through in spite of the high inductance of the ma- 
chine. The first step toward this end would evidently be to 
reduce the frequency, but evidently even at low frequency the 
losses from eddy currents in the solid iron would be serious, 
and the next obvious step is to construct the field as well as 
the armature of iron laminated like a transformer core. In 
fact it has been known for a long time that a series-wound 
motor with a laminated field would operate after a fashion 
when fed with low frequency alternating currents, say at 8 or 
10 periods per second. The recent work has been in the direc- 
tion of so specializing this machine as to keep down the in- 
ductance and to reduce the sparking to reasonable limits when 
operating at the lower commercial frequencies. 

The chief electrical feature of the a.c. series motor is that its 
total counter E. M. F. is the geometrical sum of the E. M. F.'s 
induced by the motion of the armature conductors and those 
due to reactance in the armature and field respectively. Now 
the apparent watts supplied are measured by the product C E^, 
where E^ is the impressed E. M. F. while the useful energy is 
determined by E, the motor E. M. F. as in any other motor. 
Hence, in order that an a.c. series motor should have a good 
power factor and apparent efficiency, it is necessary to make 
E large compared with the reactances of armature and field. 
To do this the number and the speed of the armature wires 
may be increased on the one hand and the reactances kept 
down upon the other. The first condition points to a motor 
having a relatively simple field and a very powerful high speed 
armature, while the second condition calls for low frequency 
and very careful designing against reactance. 

To reduce the field reactance the turns on the field must 
be kept low, since one cannot reduce the effective field magnet- 
ization without reducing that in the armature also, and to 
maintain the field with few turns, requires a small air gap of 



264 



ELECTRIC TRANSMISSION OF POWER. 



large area. Since one cannot reduce the air gap beyond a 
certain point without mechanical difficulties, and cannot 
increase its area much without increasing the general dimen- 
sions of the motor, the saving of reactance in the field is neces- 
sarily rather limited. 

One can, however, considerably reduce the armature reac- 
tance by winding a neutralizing coil so as to surround the 
revolving armature in a plane approximately perpendicular 
to the line joining the brushes. This is known as a compen- 
sating coil and the motor fitted with it as a compensated 
series motor. Fig. 157 shows this arrangement in diagram. 
The result is to very greatly diminish the net armature reactance 

so that the power factor may be 
carried to .90 or even more. Here 
A is the armature, F the field, and 
C the compensating coil. 

The recently introduced single- 
phase motors for electric traction 
generally belong to this type, and 
in certain cases these machines 
may be useful for variable speed 
work on commercial circuits, for 
they behave under supply at varied 
voltage quite like d.c. series motors 
and indeed can be worked on d.c. circuits. Fig. 158 shows a 
Westinghouse single-phase railway motor with the armature 
removed, showing the field coils and the compensating coils. 
A modification of the same idea is shown in Fig. 159, where 
the compensating coil is short circuited, the motor being other- 
wise arranged as before. Still another commutating type of 
a.c. motor is that shown diagrammatic ally in Fig. 160 in which 
there are field and compensating coils in series, but the arma- 
ture is short circuited upon itself. This is substantially like 
Prof. Elihu Thomson's repulsion motor in the original form 
of which, however, the coils F and C were replaced by a small 
resultant coil in an intermediate position with respect to the 
brush line. This motor too has been developed for railway 
work, and has much the same properties as the regular series 
compensated type. 




Fig. 157. 



ALTERNATING CURRENT MOTORS. 



265 



All these alternating motors have good power factors when 
working near their normal speeds, often rising to .90 and 




Fig. 158. 



more, but their efficiency is generally materially less than that 
of (I.e. motors, or polyphase induction motors of similar out- 
put. The losses from the more complex windings, from eddy 





Fig. 159. 



Fig. 160. 



currents and from hysteresis are enough to cut down efficiency 
generally 5 to 10 per cent, more often near the latter figure. 

By careful design of the commutation sparking can be kept 
within reasonable bounds, at least within a moderate range 



266 



ELECTRIC TRANSMISSION OF POWER. 



of speed, although the conditions for sparkless operations can 
never be as favorable as in d.c. motors. 

Several other modifications of the commutating a.c. motor 
have been devised, but they all depend on principles similar 
to those already mentioned, and may be expected to perform 
in about the same way. Kone of them can reasonably be 
expected to do materially better than the series compensated 



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Fig. 161. 



8U 90 100 110 130 H.P.1 



type first described. They are one and all intended for low 
frequency work generally 25 ^ as a maximum. The higher 
the frequency the harder to build a good commutating motor. 
Hence, whatever place such motors may find in traction, they 
will probably be of rather limited applicability on ordinary 
commercial circuits of the present usual frequency of 60^, 
although they may be occasionally useful. 

The practical properties of good modern induction motors 
are strikingly similar to those of shunt-wound or separately 
excited continuous current motors. 

For the same output, the induction motor generally has the 
advantage in weight, owing to the fine quality of iron which has 



ALTERNATING CURRENT MOTORS. 



267 



to be emploj^ed, but its laminated structure and rather com- 
plicated primary winding make it fully as expensive to build, in 
spite of the absence of a commutator. 

In point of commercial efficiency there is but little differ- 
ence. It is not difficult to build an induction motor which is 
fully up to the average efficiency of other motors of similar 
output and speed. And what is of greater importance, the 
question of sparking being eliminated, the point of maximum 
efficiency can quite easily be brought somewhere near the aver- 



50 H.P.|tWO phase JTESLA MOTOR 
220 V0UTS-3000 ALTS. -750 R.P.M. 




30 40 50 CO 

OUTPUT-BRAKE H.P. 

Fig. 162. 



age load. It must be remembered that here, as elsewhere, 
the last few per cent of efficiency ate somewhat costly, and not 
always found in the rank and file of commercial machines. 

The weak point of commercial induction motors is apt to be 
the power factor. Of course low power factor means demand 
for current quite out of proportion to the output, and hence 
greater loss in the lines and greater station capacity. In 
addition, a heavy lagging current makes regulation of voltage 
on the system anything but easy. 



268 



ELECTRIC TRANSMISSION OF POWER. 



Now, it is perfectly feasible to build induction motors with 
power factors so high as to avoid these practical difficulties 
almost entirely. But this result is somewhat expensive, 
whether reached by finesse in design, or by the addition of con- 
densers, and it is therefore not always attained. 

Slow speed induction motors, large and small, are subject 
to bad power factors, and so in fact are all induction motors 
having many poles. The best results, however, are ver}?- good 
indeed. A power factor of .9 or thereabouts at normal load 




10 1^ U 16 18 20 22 21 
MECHANICAL HORSE POWER 

Fig. 163. 



is quite unobjectionable in practice, and this figure can be 
reached or closely approximated by careful design. 

In point of efficiency there is little difficulty in reaching 
satisfactory figures. The actual properties of polyphase induc- 
tion motors can be best appreciated by the examination of 
their characteristic curves, showing the variations of efficiency, 
power factor, and speed under varying loads. Fig. 161 shows 
these curves for a 75 HP three-phase motor built by the General 
Electric Company. It is a 60^ motor, intended for severe ser- 
vice, and hence is arranged to carry considerable overload at a 



ALTERNATING CURRENT MOTORS. 



269 



good efficiency. The fall in speed from no load to full load is 
but 3 per cent, and the starting torque is 80 per cent greater 
than full running torque, with an expenditure of current closely 
proportional to the torque. The commercial efficiency reaches 
91.1 per cent, and the power factor 84.3 per cent, which is not 
bad for so large a motor intended for considerable overloads. 
Fig. 162 shows the characteristics of a Westinghouse two- 
phase induction motor of 50 HP for 25^. Its properties, as 
might be expected of a well-designed motor for so low a fre- 




10 H.P,) 



3000 ' 1000 
Fig. 164. 



fiOOO BRAK£iVAria^ 



quency, are admirable, particularly the great efficiency at 
small loads. 

Fig. 163 shows the properties of a polyphase motor of 20 
HP at 130^, used with Stanley condensers to keep down the 
results of the inductance encountered at so high a frequency. 
The effect of this device, particularly at moderate loads, is 
very striking indeed. Without condensers one could not 
obtain such a power factor even at full load. While the con- 
denser does not perfectly compensate for inductance, it does 
so sufficiently well for all practical purposes. In other prop- 
erties the motor is not so especially remarkable. 

These curves are from the manufacturers' tests, and the 



270 



ELECTRIC TRANSMISSION OF POWER. 



author believes them to be entirely trustworthy, although they 
probably rei^resent good results. Better curves than these are 
occasionally obtained, generally for some individual reason. 
Now and then a ''freak" motor is produced, with enormously 
high efficiency or power factor, like a certain 5 HP three-phase 
motor designed and tested by the author, which gave at full 
load a power factor of .94. 

On the other hand, it is unfortunately true that many com- 
mercial induction motors are not as good in point of efficiency 
and power factor as they ought to be. A series of tests of 
induction motors under the direction of Professor D. C. Jack- 
son was published a few years since, which gives data so instruc- 
tive and impartial as to be well worth reproduction here. The 
motors tested were, except for a 10 HP Westinghouse two- 
phase, all of 5 HP nominal capacity, and by the following 
makers: Westinghouse, Fort Wayne Electric Corporation (syn- 
chronous self -starting monophase), Stanley, Allegemeine Elec- 
tricitats Gesellschaft, General Electric Company. In addition, 
results of tests on Oerlikon and Brown motors are included in 
the results. Fig. 164 shows the efficiency curves and regu- 
lation of the several machines, and the table gives a general 
view of their respective properties. 

COMPARATIVE QUALITIES OF INDUCTION MOTORS. 





ci( 


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Cent of Fun 

Load. 




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Cent. 


Power Factors, Per 
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5 


140 


131 


77 


157 


3 


77 8 


55,2 


70.6 


77 


76.6 


76.4 


13 


37.5 


50 


62.3 


71.5 


2 


5 


5 


100 


45 


44 


153 




83,8 


54.5 


72.5 


80.5 


83.8 


67.5 


10 5 


25 5 


58 


67.3 


64 


3 


5 


6.26 


104 


90 


83 


114 


9,7 


79 


52 


71.9 


78.9 


77 


84 


74 


81.6 


83.3 


83.7 


84 


4 


5 


5 


186.5 


136 


39.4 


262 


7 


79.5 


62,2 


77.2 


79.5 


77.8 


82.6 


15.5 


44.7 


62.5 


73.7 


80.3 


5 


6 


6 55 


171 


138.5 


86.6 


156 


3.7 


88 


75,2 


85 


87.7 


88 


78.5 


6.8 


27.5 


45.5 


59.8 


70.3 


6 


10 


10 


147 


142 


137 


194 


4.5 


82 


67 


78.4 


81.9 


81.1 


80 


15 


40.4 


59.1 


69 


73.4 


7 


4.5 


4.5 


163 


232 




357 


3 7 


79 1 


61 


73 8 


78 4 


79,1 


88 4 


22.2 


51.3 


69.4 


80.3 


85.2 


8 


5 


5 










4.6 


83.8 


65.5 


80.2 


83.8 


82 


89 




52 


71.5 


81.7 


87.2 



Note. — 6, 7, and 8 were not run up to maximum load, on test. 



ALTERNATING CURRENT MOTORS. 271 

Looking over these results, Nos. 3, 5, and 8 are decidedly 
the best of the lot. Of these, No. 3 is possessed of a fairly high 
and very uniform power factor, but rather moderate efficiency. 
It starts well, and with a moderate current has sufficient 
margin of capacity for all ordinary work, but its speed falls 
considerably under load. No. 5 has extraordinary efficiency at 
all loads, starts admirably, and can carry a tremendous over- 
load — more than double its rated capacity. Moreover, it 
regulates very closely. The power factor, however, is so bad 
as to be a curiosity, having apparently been sacrificed to 
obtain great maximum output, which is for many purposes use- 
less. No. 8 is a far better all-round machine than any of the 
others, has a good maximum efficiency at a little below full 
load, and an excellent power factor. Professor Jackson notes 
that since, at an output of 3^ HP, No. 3 has an efficiency of 
75.5, and a power factor of 83-^, while No. 5 shows respec- 
tively 85 and 59, the station capacity for the latter must be con- 
siderably greater than for the former. That is, the apparent 
efficiency of Nq. 3, which determines the necessary station 
capacity, is 64 per cent, while that of No. 5 is 50 per cent. 
Hence, to supply one brake HP with No. 5 motors, there must 
be a station capacity of 2 HP, while with No. 3 motors 1.56 HP 
is sufficient. But with No. 8 the efficiency is about .83, and the 
power factor about .80, giving an apparent efficiency of .66, 
which is better than either No. 3 or No. 5. Motors like 
No. 3 are excellent for the power station, but hard on the 
customer, while No. 5 is admirable for the customer, but bad 
for the station. No. 8 is fair to both parties. 

Most of the motors shown start quite well enough for ordi- 
nary purposes. Neither heavy starting torque nor ability to 
carry large overloads is needed in ordinary motor work. 
Large torque per ampere is, however, desirable. It is best 
secured by using at starting a non-inductive resistance in the 
secondary circuit as found in many existing motors. The 
actual effect of this resistance is as follows: It reduces the 
current drawn from the mains so that the motor will not 
seriously disturb the voltage on the lines at starting; by dimin- 
ishing the current flowing in the armature it limits the arma- 
ture reaction so that it may not beat back the field so as to 



272 ELECTRIC TRANSMISSION OF POWER. 

interfere with proper starting, nor distort it so as to produce 
dead points; and, finally, it largely increases the torque per 
ampere, which greatly aids in starting under load. 

The function first mentioned is very important where lights 
and motors are to be operated, since if a motor is capable of 
starting under heavy load it is likely to take at starting a 
pretty large current, which may pull down the voltage in the 
neighborhood merely in virtue of ohmic drop. Besides, the 
power factor of an induction motor at starting is only about 
.7, so that the heavy current lags severely and still further 
interferes with proper regulation. 

The heavy lagging current set up in the armature is likely 
to distort the field seriously, sometimes so much as to block 
the starting of the motor, sometimes merely producing dead 
points, i.e., points of no torque, or greatly weakening the 
torque in certain positions of the armature. The introduc- 
tion of resistance in the secondary circuit both diminishes the 
current and its angle of lag, and thus keeps down the arma- 
ture reaction. In some motors the reluctance of the magnetic 
circuits is sensibly the same in all angular positions of the 
armature, so that there are no points of noticeably weak 
torque either with or without a starting resistance. But 
som.e motors otherwise excellent have sufficient variations of 
reluctance to produce bad dead points when the armiature 
reactance is severe, while these nearly or quite disappear by 
adding resistance in the secondary circuits. 

The use of resistance in the secondary at starting obviously 
throws forward the phase of the secondary current so that it 
is in better relation to the field magnetization, and hence 
although the numerical value of the current is reduced, its 
effective component is increased. The considerations which 
affect the relations between torque and current in the arma- 
tures of induction motors are in reality quite simple. The 
absolute value of the current, other things being equal, is 
determined by the armature impedance, and is the same for 
the same impedance whatever the relation between the reac- 
tance and resistance components of that impedance. The 
ratio between these components, however, determines the 
phase angle of the armature current, so that for a given value 



ALTERNATING CURRENT MOTORS. 273 

of the current the torque depends on the ratio between resis- 
tance and reactance in the armature. 

By lessening either the resistance or reactance of the arma- 
ture a motor is obtained in which a very large current flows 
at starting, but reducing the impedance by cutting down 
reactance gives the resulting current a better phase angle 
than that obtained by reducing resistance alone. For a given 
motor the maximum torque is obtained when the ratio of 
resistance and reactance is unity, i.e., when 

I = R. 

Now, one can cut down the resistance by increasing the 
allowance of armature copper, and can diminish the reactance 
by subdividing the winding so that there shall be many slots 
in the armature, and the minimum possible number of turns 
per slot. Also the better the mutual induction between field 
and armature the less the reactance of either member is likely 
to be, so that by close attention to design it is possible greatly 
to reduce the armature reactance. In commercial motors the 
relation between resistance and reactance in the armature is 
generally from 

/ = 3 i^ to / = 10 ie. 

Hence, when large torque per ampere is desired the simplest 
thing to do is to insert non-inductive resistance in the secon- 
dary, and when 

I = R 

the given motor will be at its best with respect to starting 
torque. With 

I = R 

the maximum torque will be obtained when both are as small as 
possible. Hence, if very great starting torque is desired, the 
motor should be designed with very low armature resistance 
and reactance. 

The slip of the motor below synchronous speed depends 
upon the armature resistance in induction motors, just as in 
continuous current motors the slip below the speed at which 
the armature would give the impressed E. M. F. is determined 



274 ELECTRIC TRANSMISSION OF POWER. 

by armature resistance. In each case the shp measures the 
percentage of energy lost in the armature, so that if an induc- 
tion motor, for example, runs loaded at 5 per cent slip the loss 
of efficiency in the armature is 5 per cent. 

Commercial induction motors vary widely in slip — from as 
little as 1 per cent to 8 or 10 per cent, according tc design. 
It must not for a moment be supposed, however, that small 
shp imphes high efficiency of the motor. One can put, in 
designing a motor, most of the loss into the armature or into 
the field, as one pleases, and it is pretty safe to say that if there 
is remarkably little in the armature there will be an unusual 
amoimt in the field, unless cost is utterly disregarded. Prob- 
ably the best all-around results can be obtained by dividing 
the permissable loss nearly equally between armature and 
field. 

There is a very simple relation between the static and run- 
ning torques of an induction motor, the static and running 
currents, and the slip, as follows : 

T^= S — • 

In this equation Tg is the static torque, Cg the static current, 
Tg and Cg torque and current of the slip S, and S that slip 
expressed as a percentage. As an example of the application 
of this formula, suppose the full load current of a certain 
motor is 60 amperes per phase, the current with the armature 
at rest 400 amperes, and the slip at full load is 5 per cent. 
Then 

^_05xi55^=2.22, 
Ts 3600 

i.e., the static torque will be 2.22 times the full load running 
torque. Of course, if a motor is to have a powerful starting 
torque it must take a pretty heavy current, but the extra 
resistance at starting helps very materially in keeping the 
current within bounds. An adjustable secondary resistance 
makes it easy to bring to speed any load that the motor will 
carry continuously, without demanding excessive current. 
As to overload, an ability to carry 25 per cent more than 



ALTERNATING CURRENT MOTORS. 



275 



the rated capacity is ample, save in rare cases, and greater 
margin than this usually means some sacrifice in efficiency 
or power factor at normal loads. For most work an effi- 
ciency curve hke that of No. 8 is preferable to one Uke that 
of No. 5. When great margin of capacity is needed, it is best 
to use a motor dehberately adjusted to such use, and not to 
expect it of a motor properly designed for ordinary service. 
The speed of induction motors is best regulated by inserting 



13 

11 




























y 


A 


























/ 


y 




e 
























y 


y 




c 






















■r^ 








a- 


8 

1' 

i 










• — 




/ 






















y 


/ 
























y 


/ 
























y 


/ 


















.1 








y 


/ 




















A 


E 
< 






/ 


^ 






















s 






/ 


r 
























1 


/ 






























i 































e 6 7 8 9 

Speed in 100 r.p.m. 

Fig. 165. 



10 



12 13 14 ta 



a non-inductive resistance in the secondary circuit. Under 
these circumstances the motor can be miade to run at con- 
stant torque over a very wide range of speeds by varying the 
resistance, just as one would regulate a street-car motor. 
Fig. 165 shows the variation in speed, current, and power 
factor in a 15 HP three-phase motor fitted with rheostatic 
control. The speed was varied at constant torque from about 
1,400 r. p. m. down to 150 r .p. m. Curve B shows the variation 
of the power factor, in this case high at all speeds, and curve 



276 ELECTRIC TRANSMISSION OF POWER. 

C shows the shght variation in input. Operated in this way, 
the motor behaved almost exactly like a series-wound direct 
current motor with rheostatic control. Such a rheostat is 
used in operating hoists and the like with induction motors. 
Regulation by varying the primary voltage is highly unsatis- 
factory, since the torque falls off nearly in proportion to the 
square of the voltage, so that at low speeds the output is 
enormously reduced. Regulation by any method involving 
resistance is of course inefficient, not materially more so, how- 
ever, than in the case of continuous current motors. It should 
be understood that all these remarks concerning torque, 
regulation, and the like apply to polyphase induction motors 
and do not hold true in general of monophase motors. 

The weak point of induction motor practice is in the heavy 
inductance likely to be encountered unless motors with first- 
class power factors are used. It is depressing to find the 
current capacity of your generator exhausted long before it 
has reached its rated output in kilowatts, and if the motor 
service is part of a general system, the effect of a bad power 
factor on regulation is disastrous. 

With generators of mioderate inductance and good motors, 
general distribution by polyphase currents gives admirable 
resiiltSo The station manager should see to it that his motors 
are not of excessive size for their work, and are good in the 
matter of power factor. A few motors for very variable loads 
can be handled readily enough, but no motor with a bad 
power factor should be tolerated simply because it is cheap. 
Power factors of at least .85 at full load, and .80 at two-thirds 
load, are quite obtainable except in case of some special motors, 
and should be insisted upon rigorously. 

One polyphase station operating more than fifty induction 
motors showed, when tested by the author, about .65 as aver- 
age power factor when carrying all the motors. Rigorous 
inspection of the motors installed would have raised this 
figure to .75, although the existing power factor actually gave 
no trouble, there being ample generator capacity. 

So much for polyphase induction motors. Monophase 
motors generally fail to give so uniformly good results. Oc- 
casional extraordinary results have been reported from the 



ALTERNATING CURRENT MOTORS 277 

latter, but in the author's opinion they concern motors which 
belong to the ''freak " class alluded to^ and cannot be expected 
in commercial practice. Monophase motors are usually weak 
in power factor save at certain loads, and start badly, most 
of those in use abroad being started without load. Even so, 
the starting current is large, as may be safely concluded from 
the discreet silence preserved on this topic in all descriptions 
of monophase motor installations. In general power trans- 
mission work the incandescent lamp and the induction motor 
are the chief factors. Synchronous motors are valuable in 
their proper place, and arc lighting and continuous current 
work are sometimes relatively important. The alternating 
current systems are now far enough developed to be entirely 
workable and trustworthy for incandescents and motors. 
The alternating arc lamp is, however, not quite in condition 
to replace the continuous current arcs for all purposes and 
under all circumstances, and for work specially suited to con- 
tinuous currents reliance has at present to be placed in various 
current reorganizing devices, which are, so far, of rather 
indeterminate ultimate value. Whether they are to have a 
large permanent place in the art, or whether their sphere will 
gradually be much contracted, is uncertain. At all events 
it is sufficiently clear that the main body of power transmission 
will have to depend on alternating currents, at least for a long 
while to come. 

Even if continuous current should be obtained somewhat 
directly from coal in the near or far future, the result would be 
not to increase power transmission by continuous currents, 
but to render the transportation of coal by far the cheapest 
method of transmitting energy. 

The relative importance of polyphase, heterophase, and 
monophase systems is a question often raised. The present 
indications are that the polyphase systems, in virtue of in- 
creased output of generators, possible economy in copper and 
general convenience, have come to stay. The monophase 
motor problem has not yet been satisfactorily solved in any 
general way, and until it has been solved, the monophase sys- 
tem must remain subordinate, like the heterophase systems, 
which are special rather than general in their applicability. 



278 ELECTRIC TRANSMISSION OF POWER. 

The much mooted question of frequency will be referred to in 
its different bearings in connection with other topics. The 
frequencies once common, 120^ to 135^, are rapidly passing 
out of use for all important work. They are inconveniently 
high for long lines by reason of inductance, are troublesome 
for large units, lead to high inductance in the system, and 
have for their only compensating advantage, lessened cost of 
transformers. Both here and abroad lower frequencies have 
come into use. In this country 60-^ seems to be the favorite 
frequency, except for work with rotary converters, when 25~ 
to 35^ is usual. Both these last are too low for general 
practice, since the cost of transformers is greatly increased; 
the former is unsuitable for incandescent service, unless with 
extremely low voltage lamps, and both are unsuitable for 
alternating arcs. It is now pretty generally recognized for 
the above reason, that the adoption of so low a frequency as 
25^ in the great Niagara plant was an error of judgment, 
perhaps brought about by an overestimate of the importance 
of rotary converters in general distribution. The only appa- 
ratus which at present demands low frequency is the single- 
phase commutating motor. Should it come into great use 
plants of 25^ or less may be necessary, but for general dis- 
tribution it is always preferable to keep the frequency high 
enough for incandescent lamps, which are the most profitable 
kind of load. 

On the other hand, abroad a compromise frequency of 40^w 
to 50^ is in general use. In the author's opinion there are 
very few cases in which lower frequencies than these are 
desirable, and none in which less than 30^^ should be toler- 
ated for general distribution work; 50^- or 60^- meets general 
requirements admirably, and only in rare cases is the use of 
rotary converters of sufficiently commanding importance to 
call for a lower frequency. 

In connection with this topic we may consider a verbose 
controversy which has raged of late, respecting the advantages 
of certain irregular forms of alternating current waves vs. a 
true sine wave. The facts in a nutshell are as follows: Cer- 
tain complex current waves, whose irregularity is due to the 
presence of harmonics of higher frequency, have been found 



ALTERNATING CURRENT MOTORS. 279 

to give slightly better efficiency in transformers than sine 
waves of the same nominal frequency. Such waves, however, 
do not hold their form under varying conditions of load, and 
by reason of their harmonics of higher frequency raise the 
inductance of the line and apparatus, increase the probability 
of resonance on the line, hamper all attempts to balance the 
inductance of the system by condensers or synchronous motors, 
and finally sometimes interfere with the proper performance of 
induction motors. The use of such wave forms, then, is likely 
to lead to very embarrassing comphcations in a power trans- 
mission system, and their sole advantage is far better secured 
by using a sine wave of slightly increased frequency, than by 
interpolating a set of worse than useless harmonics. 

It is needless to say that all cases of power transmission 
cannot be treated alike — there is no system that will meet all 
conditions in the best possible manner. The best results will 
be obtained by treating, in the preliminary investigation, 
each problem as an unique and independent case of power 
transmission, and afterward boiling down the conclusions to 
meet practical conditions. Avoid, when you can, apparatus 
of peculiar sizes and speeds — remember that you are after 
results, not electrical curios. See to it that what is done is 
done thoroughly, and for general guiding principles keep your 
voltage up and your inductance dowTi, and watch the line. 



CHAPTER VII. 

CURRENT REORGANIZERS. 

Whatever method may be emploj^ed for the transmission 
of power in any given case, it will often be found that the 
current delivered at the receiving station is not of the charac- 
ter needed. Sometimes in transmissions for special purposes 
no difficulty will be met, but frequently, especially in the 
transmission of power for general distribution, both continu- 
ous and alternating currents are needed, whereas only one is 
at hand. For all electrolytic operations, for most railway 
work at present, for telegraphy, and sometimes for arc light- 
ing, continuous current is necessary, while alternating current 
is necessary for convenient application to electric furnaces, 
electric welding, electro-cautery and other minor purposes. 
So whichever kind of current is transmitted the other must be 
derived from it for certain uses. 

All devices for thus changing alternating to direct currents, 
or vice versa, with or without accompanying change of voltage, 
may properly be called current reorganizers. 

Three classes of such apparatus have come into considerable 
use: 1. Commutators; 2. Motor dynamos; 3. Rotary con- 
verters. These classes are quite distinct from each other; 
each has advantages and faults peculiar to itself, and all three, 
especially the last named, are in every-day practical use to a 
greater extent than would seem probable at first thought. 

We have already looked into the matter of commutation in 
Chapter I, and have seen how the naturally alternating cur- 
rents in a continuous current dynamo are rectified and 
smoothed. Given, then, an alternating current received from 
a distant generator, and it would seem an easy matter to 
receive this current upon a commutator and deliver it as con- 
tinuous current. In point of fact there are very serious diffi- 
culties in this apparently simple process. 

The current received is a set of simple alternations shown 

280 



CURRENT REORGANIZERS. 281 

diagrammatically in Fig. 166. The figure shows three complete 
periods. Now, if such a current be sent into a simple two-part 
commutator, such as is sho^vn in Fig. 9, Chapter I, revolving 
at such a speed that the brushes will be just passing from one 
segment to the other every time the current received changes 
direction, the result will be a rectified current, shown in Fig. 
167, unidirectional, it is true, but far from continuous. Vari- 

/iilk jiiik jjiii^ 
^^^0 Nipj^ ^lip7 

Fig. 166. 

ous modifications of this simple rectifying apparatus have been 
and are in extensive use for supplying current to the field 
magnets of alternating generators. As these machines are 
generally multipolar, the two-part commutator has been modi- 
fied so as to reverse the current at each alternation. Fig. 
168 shows one of the simple forms of commutator arranged 
for self-exciting alternators. It consists of a pair of metal 
cylinders mounted on and insulated from the dynamo shaft. 
Each cylinder is cut away into teeth, and the two are mounted 
so that the teeth interlock with insulation between them. 
Each pair of consecutive teeth acts like the ordinary two-part 
commutator, and there are of course a pair of teeth for every 
pair of poles, so that the commutator acts at each alternation. 
The resulting rectified current is then led around the field 
magnets of the generator, furnishing either the whole excita- 
tion, or enough to compound the machine. Such a current, 

Ji yi%iiikiiii^^i^ 

Fig. 167. 

however, is so fluctuating that it is by no means the equivalent 
of an ordinary continuous current for magnetizing purposes, 
hence in most modern machines the main exciting current is 
furnished by a small exciting dynamo, driven from the alter- 
nator shaft or by separate means, while the rectified current is 
used only now and then for compounding. 

This simple current reorganizer is very successful for the 
purpose described. But it must be remembered that the 



282 ELECTRIC TRANSMISSION OF POWER. 

amount of energy concerned is trifling, only a very few kilo- 
watts being required to compound even the largest alternators. 
And despite this, there is often trouble from sparking, such 
commutators being notoriously hard to keep in good order. 

In applying the same process to rectifying currents on a larger 
scale, the difficulties from sparking are very serious, in fact 
generally prohibitive. And the worst of it is that they are 
inherent. The root of the trouble is that the alternating 
current on a line used for general purposes cannot be kept 
accurately in step with the motion of the commutator. To 
ensure sparkless commutation the conditions must be as shown 
in Fig. 169. 

The alternations of the current and E. M. F. are shown by the 




Fig. 168. 



solid line, while the brushes at the moment of passing from one 
commutator segment to the next must take the position h h, 
with respect to the current. That is, they must pass from one 
segment to the next at the moment when the current, just 
reversing, is practically zero. So long as the electromotive 
force and the current are in phase with each other, as shown 
in the solid line, the current will be rectified without tro\ible- 
some sparking. But when the current lags behind the 
E. M. F., as shown by the dotted line of Fig. 169, there is trouble 
at once. The brushes, as can be seen from the dotted pro- 
longations of h b, must break a considerable current, and there 
is certain to be sparking. Nor can any point be found for the 
brushes at which they will not have either to break this cur- 
rent or to pass from one segment to the next while there is 
considerable E. M. F. between segments. The case is bad 



CURRENT REORGANIZERS. 



283 



enough in a compounding commutator having a position fixed 
with reference to the E. M. F. of the machine and deahng 
with low voltage and moderate current. The inevitable result 
is sparking that can be only mitigated by shifting the brushes, 
and more or less demoralization of the compounding. If the 
current be received from a distant generator on a commuta- 
tor driven by a synchronous motor, the condition of things 
is much worse. When the current lags (or leads), not only 
are the brushes generally thrown out of step with it, but if 
there is a sudden change of phase the iiiertia of the commuta- 
ting apparatus will put it at serious variance for the time with 
both current and E. M. F. Add to this the disturbances of 
phase produced by armature reaction in both generator and 
motor, and one has a set of conditions that renders sparking 

h 




Fig. 169. 



absolutely certain. The most that can be done to help matters 
is to employ palliative measures to delay the destruction of the 
commutator. Aside from this sparking, it is nearly out of the 
question to hold the voltage of the rectified current steady if 
the phase is shifting, as it often is likely to be. 

Incidentally may be mentioned the fact that in working such 
a commutating apparatus, just as in rotary converters, the 
direction of the rectified current will be uncertain; the brush 
which happens to be on a positive segment w^hen the brush 
circuit is closed, will stay positive, as can readily be seen by 
tracing out the rectifying process in Fig. 168. In ordinary 
compoimding commutators this uncertainty is absent, for with 
the brushes in a fixed position the positive segments will 
always be under the same brush, since the segments are fixed 
with reference to the armature coils. 

No small amoimt of time and money has been spent in try- 
ing to work out a successful synchronizing commutator. The 
main trouble is, of course, sparking, and the exasperating part 



284 ELECTRIC TRANSMISSION OF POWER. 

of the problem is that while on a small scale, as in compound- 
ing alternators, fair results can be obtained, the difficulties 
increase enormously with the output, so that every attempt on 
a scale really worthy of serious consideration has ended in 
discouragement and the scrap heap. 

The great usefulness of such apparatus if of reasonably good 
qualities, has made this field of experimentation very interest- 
ing, and a vast amount of ingenuity has been expended in 
elaborately devised plans for reducing sparking and mini- 
mizing the evil results of shifting phase. An example of such 
work, of more than usual merit, was shown at the International 
Congress of 1893 at Chicago. This was the current reorganizer 




Fig. 170. 

devised by C. PoUak, for use in connection with accumulator 
installations. It was intended specifically for charging accum- 
ulators, and is very ingeniously adapted to that use. Its 
general appearance is shown by Fig. 170. The apparatus 
consists of a small synchronous motor driving a commutator, 
which has, in the example shown, eight segments coupled alter- 
nately in parallel so as to produce the effect of Fig. 168. The 
PoUak commutator is, however, peculiar in that the spaces 
between segments are of nearly the same width as the segments 
themselves, while the collecting brushes are set in pairs, so 
that by setting one of each pair ahead of, or behind the other, 
the ratio of segment width to space width can be changed. In 



CURRENT REQRGANIZERS. 285 

charging accumulators the E. M. F. of the charging current 
must always, to prevent waste of energy, exceed the counter 
E. M. F. of the battery. Hence a current rectified as in Figs. 
167 and 168 cannot successfully be used. The arrangement of 
segments just described enables the brushes to be so set that 
contact with a segment is made at the moment when the rising 
E. M. F. of the alternating side is exactly equal to the counter 
E. M. F. of the battery, and broken when the falling E. M. F. 
reaches the same value. Only that part of the current wave 
of which the E. M. F. exceeds the counter E. M. F. of the 
battery is used, the charging circuit being open during the 
remainder of the period. When well adjusted and used on 
a circuit nearly non-inductive, the machine in question is 
almost sparkless and very well adapted for the particular pur- 
pose intended. It is also highly efficient, the only losses being 
those in the motor, jylus brush friction. The total amount of 
these need be but trifling, probably less than 5 per cent of the 
output. 

But such apparatus cannot be considered as a general 
solution of the problem, for while quite successful for an 
output of 10 KW or so, it has not been tested in large sizes, 
nor under the conditions of inductance ordinarily to be ex- 
pected on a power transmission circuit. For the reasons 
already adduced the chances for success are not good, particu- 
larly since all questions of sparking become very grave when 
large currents must be dealt with. This difficulty is well 
known in dynamo working. For instance, in an arc machine 
there may be frequent recurrence of the long, wicked-looking 
blue sparks familar to everv dynamo tender, without notice- 
able damage to the commutator, while in a low voltage 
generator sparking of much less formidable appearance may 
put the machine out of business in a very short time. 

Bearing all this in mind, it is but natural to expect that 
another particular solution of the reorganizing problem might 
be fo\md for arc fighting. Here the irregularity of a ''recti- 
fied" current is of small consequence, while the small amount 
of current cannot cause really destructive sparking if other con- 
ditions are fairly favorable. So it is that we find commutating 
apparatus in quite successful use for arc lighting in connection 



286 



ELECTRIC TRANSMISSION OF POWER. 



with alternating stations. The form of apparatus shown in 
Fig. 171, designed by Ferranti, has been introduced in 
several British stations with good results. The commutating 
mechanism is of course used in connection with a '^c(mstant 
current" transformer, arranged so as automatically to hold 
the current closely uniform under all variations of load. 
Each commutating unit supplies two separate arc circuits of 
moderate capacity — twelve lights in each. How well the same 
device works at several times the E. M. F. necessary to supply 
so small a series, is now being demonstrated. The present 
tendency in central station practice is to employ very high 







Fig. 171. 

voltages for arc lighting — 50 to 100 or 125 lamps in series, 
thus greatly simplifying both the station equipment and the 
circuits. The rectifier should at least be able to replace the 
smaller generators now in use, and such machines are now built 
for as many as sixty lights. This is probably practical — in 
fact there seems to be no good reason why the rectifier should 
not be entirely available Avherever it is desirable to work 
series arc circuits in connection with a transmission plant. 
Although not in use sufhcientl}^ long to enable one to pass a 
final judgment, the machine is at least promising and worth 
careful investigation. There seems to be some doubt as to the 
successful working of these rectifiers at anything except rather 
low frequencies, 30 to 40^ or less, but such a difficulty would 



CURRENT REORGANIZERS. 287 

appear to be constructional rather than inherent. It is possi- 
ble that the alternating arc lamp will be developed far enough 
to render continuous current arcs entirely unnecessary, but this 
remains yet to be proved, although the inclosed alternating arc 
now gives highly successful results, particularly in street 
lighting. 

All rectifying commutators now in practical service are of 
very limited output — not much exceeding 10 to 20 KW, an 
amount merely trivial so far as large enterprises are concerned. 
For railway work or incandescent lighting, these very interest- 
ing machines cannot be considered in the race at present. The 
general problem is as yet unsolved by such means, useful as 
they may be for special purposes. 

The current delivered by rectifiers is in a measure discon- 
tinuous, and, hence, is not the full equivalent of an ordinary 
continuous current. The Pollak machine, however, which is 
intended to be used with a somewhat flat-topped alternating 
current wave, has been successfully employed for working 
motors as well as for charging accumulators. It is not impos- 
sible that such apparatus may yet be constructed of sufficient 
capacity to be of much practical service, although the difficul- 
ties, as has already been pointed out, are very considerable, 
and of a kind very hard to overcome. Of course, polyphase 
currents can be rectified by following the same process as with 
monophase current, and a successful apparatus would often 
find some place in transmission plants. 

The advantages of the rectifying commutator are simplicity, 
efficiency, and cheapness, particularly the last. The working 
parts are a small synchronous motor, made self -exciting (and 
self -starting) by a commutator, and one or more rectifying 
commutators driven by this motor. To obtain 100 KW out- 
put, it is not necessary, as in other forms of current reorgan- 
izers, to have a machine nearly as large and costly as a 100 
KW dynamo. On the contrary, a one or two horse-power 
motor would be amply powerful to drive the commutator, 
and the whole affair could hardly cost a quarter as much as 
a dynamo of the same capacity, besides being of greater 
efficiency, particularly at partial loads. But a hundred kilo- 
watts is far beyond the output of any rectifier that has yet been 



288 ELECTRIC TRANSMISSION OF POWER. 

put to commercial service, and even a hundred kilowatts is but a 
fraction of the output that is often desirable in a single unit. 

On the other hand, a rectifier must require at least the same 
care as a dynamo, and nmst in every practical case be employed 
in connection with reducing transformers to bring the alter- 
nating current to the right voltage. The regulation too, is 
somewhat dubious, since compound winding is out of the 
question. And the current is at best disjointed, likely to 
produce needless hysteresis, and of a character rather hard to 
measure conveniently. 

To sum up, the rectifying commutator, while quite good 
enough for certain particular purposes, has so far given no 
definite promise of general usefulness. All of the serious 
attempts to develop it on a considerable scale have ended in 
failure. It is not effectively reversible, so that the task of 
converting continuous to alternating currents is quite beyond 
it. While the cheapness, lightness, and efficiency of such 
apparatus puts it in these particulars far ahead of any other 
type of current reorganizer, the verdict of experience has so 
far been adverse in spite of these advantages, and engineers 
have been driven to other and more cumbersome devices. 

The most obvious method of deriving continuous from alter- 
nating currents, is to employ an alternating current motor in 
driving a continuous current dynamo. The two machines 
may be connected in any convenient way, by belting, clutching 
the shafts together, or by putting them in even more intimate 
connection by placing two armatures on the same shaft or two 
windings on the same core. 

The procedure first mentioned is not infrequent, particularly 
when a transmission of power plant is installed in connection 
with an existing lighting or power station. A synchronous 
motor is installed in place of the previously used engines, 
belted in any convenient way to the existing generators, 
and the operation of the station goes on as before. Further 
description is unnecessary, as the apparatus is in no way out 
of the ordinary, and not at all specialized for the conversion of 
alternating to continuous currents. As a rule such installa- 
tions have temporary, and have been replaced later by special 
apparatus worked directly from the transmission system. 



CURRENT REORGANIZERS. 289 

A more interesting way of accomplishing the same result is 
by the use of a twin machine comprising motor and generator 
on the same bed plate, or even on the same shaft. In this way 
the reorganizing apparatus is formed into a compact unit, 
convenient to install and to operate, and possessing an effi- 
ciency higher than that of two belted machines, by the belt 
losses and more or less of the bearing friction. The total 
increase of efficiency is perhaps 5 per cent, when the com- 
parison is between a pair of coupled machines and a pair 
iirectly belted, or more if the belting be indirect. Moreover, 



Fig. 172. 



the motor and dynamo parts of the. machine can each be 
designed so as to give the best efficiency and economy of 
construction possible at the given mutual speed. A unit of 
this class is shown in Fig. 172 — an early Siemens continuous 
alternating transformer. The motor part is wound for 2,000 
volts, monophase, and the dynamo part, of the well-known Sie- 
mens internal pole-type, with overhung armature and brushes 
directly on the windings, delivers continuous current at 150 
volts. In this case the machine has three bearings, although 
in many cases it would be quite possible to get along with 
two. The main advantage of this duplex form of machine is 
the complete independence of the two component parts in 



290 ELECTRIC TRANSMISSION OF POWER. 

their electrical relations. The motor part can be designed 
for any desired voltage or number of alternations. It can 
often, except in very long transmissions, take the line voltage 
directly without need for reducing transformers, while the 
number of alternations can be chosen solely with reference to 
general conditions and without considering the direct current 
end of the machine at all. This, as will be seen when we have 
considered some other types of current reorganizers, is a very 
valuable property, since it gives the power of obtaining con- 
tinuous current in a thoroughly practical way from alternating 
currents of any frequency. Other reorganizers can be worked 
to advantage only within a somewhat limited range of fre- 
quency. Again, the motor dynamo can be compounded on 
the continuous current side without in any way reacting upon 
the alternating circuit, and the two circuits can be regulated 
independently in any desired manner. All difficulties due to 
lagging current can be eliminated, and the continuous current 
side can be kept at constant pressure irrespective of loss in 
the main line or any variations of voltage or phase occur- 
ring in it. 

Finally, tlie apparatus can as readily give alternating current 
from continuous, as the reverse, and with the same indepen- 
dence in each case. 

The compensating disadvantages are high first cost and 
rather large loss of energy in the double transformation. As 
to the former count, it may be said that the advantages gained 
in possible range of frequency and flexibility in the matter of 
voltage go far to offset the increase of cost. Often such 
a motor dynamo is the only possible way of securing the 
necessary current. For example, if one wished continuous 
current for heavy motor service, such as hoists and the like, 
where the only current available was monophase alternating 
of 125^, or even of 60^ for that matter, the motor dynamo 
would be the only practical way of solving the problem. 

As regards efficiency the motor dynamo should be, and is, 
a little better than motor and dynamo separately, owing to 
lessened friction of the bearings. Its efficiency should be as 
great as 85 per cent at full load, and might easily be 2 or 
3 per cent higher, in large machines. At half load it should 




Fig. 1, 




Fig. 2. 



PLATE IX. 



CURRENT REORGANIZERS. 



291 



be say 82 to 85 per cent. Practice too often shows results 
several per cent below those mentioned, but this is because 
motor dynamos have usually been of very small size and 
sometimes have been made up from any machines of the right 
speed that were at hand. 

The usual synchronous motor may in small motor genera- 
tors be replaced to advantage by an induction motor, which 
is simpler than the synchronous form and requires no brushes. 
Such a combination is shown in Fig. 173. This machine is 




Fig. 173. 



specially designed for furnishing charging current for auto- 
mobile batteries. Through most residence districts only alter- 
nating current is available, and the convenience of such an 
apparatus is ver}' great. 

The motor is a monophase induction machine of the class 
showTi in Plate VIIT, Fig. 2, suited to ordinary lighting circuits. 
Of late such machines have assumed considerable importance, 
and many large units have been produced. Plate IX shows in 
Fig. 1 a 500 KW quarter-phase set running at 400 r. p. m. It 
consists of an 8 pole 500 KW railway generator coupled directly 
to a 20 pole 2,200 volt synchronous motor, the two machines 



292 ELECTRIC TRANSMISSION OF POWER. 

having a common bearing between them. An interesting 
feature of this set is the exciter mounted on the same shaft, an 
8 KW multipolar generator, so that the whole outfit is self- 
contained. The frequency in this case is 66^, a periodicity at 
which such motor generators have a material advantage over 
other apparatus for a like purpose. 

Fig. 2 is out of the ordinary in that the motor is of the 
induction type, instead of the ordinary synchronous machine. 
The set shown is of 100 KW output, and comprises an ordinary 
6 pole 600 volt railway generator coupled to a 12 pole three- 
phase induction motor, running at 600 r. p. m., the periodicity 
being 60^. Induction motors have recently come into consid- 
erable use in this sort of work, in spite of somewhat lower effi- 
ciency than the corresponding synchronous motors. It is safe to 
say that the difference in efficiency is 2 or 3 per cent, and while 
the synchronous motor ma}^ be overexcited so as to improve 
the power factor of the system, the induction motor always 
introduces lagging current. Yet a number of motor generators 
with ioduction motors are now being built of capacity from 500 
to nearly 1,000 KW. The real reason for the use of induction 
motors on so large a scale is the trouble which has been experi- 
enced at many times and places from hmiting. These troubles 
do not get widely advertised outside the stations where they 
occur, but it is a fact that in the use of rotary converters and 
synchronous motors on a large scale very serious and formi- 
dable developments of this phenomenon have occurred, so that 
in spite of the use of shields it has under certain conditions, 
especially when incandescent lighting circuits were to be fed, 
seemed wise to have recourse to induction motors. It is, how- 
ever, probably best to regard this as a temporary expedient, 
as synchronous motors, at least, can be practically freed from 
hunting by proper design and construction, and possess very 
considerable advantages. The demand for machines of extreme 
multipolar construction, a demand based largely on fashion, 
and the use of laminated pole pieces, are responsible for a 
good share of the trouble. Rotary converters, as we shall pres- 
ently see, present even more serious problems. 

In these large motor dynamos it is possible to reach full load 
efficiencies in the neighborhood of 90 per cent, and figures 



CURRENT REORGANIZERS. 



293 



fully up to that point have actually been obtained. As large 
synchronous motors can readily be wound for 10,000 or 12,000 
volts, under favorable conditions motor dynamos can be used 
without reducing transformers, which averts a loss of 2.5 or 3 
per cent, that would otherwise be incurred. 

From the duplex machines just described it is but a short 
step to the composite dynamotor, so called, of which the 
armature is double wound. The primary or high voltage 
winding may of course be either alternating or continuous. 




Fig. 174. 

The secondary winding is likewise for either current, and may 
well be fitted with both commutator and collecting rings. 
A favorite arrangement of the windings is to place the 
secondary coils in slots in the armature core, apply a sheath- 
ing of insulation, and then to wind the primary coils on the 
smooth surface thus formed. The commutators or rings 
are placed one at each end of the armature, as in the con- 
tinuous current transformer shown in Fig. 37, Chapter III. 

A typical dynamotor of this sort is shown in Fig. 174. This 
is specifically intended to derive a high voltage alternating cur- 
rent for testing purposes from a low voltage continuous cur- 
rent. The output is small, only a fraction of a kilowatt 



294 



ELECTRIC TRANSMISSION OF POWER. 



and the armature is in the ordinary bipolar field used for small 
motors. The motor or primary winding is for 110 volts, 
continuous, and the secondary for 5,000 volts, alternating. Of 
course these voltages might be anything desirable, since in so 
small a machine there are no difficulties in the way. 

Another excellent specimen of the same type is Fig. 175, a 
Lahmeyer "umformer'' of about 30 KW output. It is pri- 
marily a continuous current transformer, with 675 volts primary 
and 115 volts secondary. It is fitted, however, as shown in 




r ^::ss 






Fig. 175. 



the cut, with collector rings outside one of the bearings, from 
which three-phase current at about 70 volts can be taken. 
There are four field poles, and as the normal speed is 850 
revolutions per minute, the three-phase current is at a fre- 
quency of a little less than 30^ per second. 

This was one of the machines exhibited at the Frankfort 
Exposition of 1891, and fortunately an efficiency test of it is 
available, dealing, however, only with continuous currents. 
From the nature of the case the efficiency with a three-phase 
secondary would not differ substantially from that found, so 



CURRENT REORGANIZERS. 



295 



that the curve, Fig. 176, gives a closely approximate idea of the 
general efficiency of such apparatus in the smaller sizes. At 
full load the commercial efficiency is very nearly 85 per cent, 
while at half load it has dwindled to 77 per cent. This is not 
bad for a small machine, and in a unit of 100 KW or more could 
undoubtedly be raised several per cent. It should be at least as 
high as can be obtained from a duplex motor dynamo, in fact 
rather higher, since the bearing friction and core losses are 
diminished. The composite machine is also cheaper, since but 
one field is used, and it has a certain advantage in that the arma- 
ture reactance due to the motor and dynamo windings tend to 



CS 




^z- 




p 








/ 





..3 



.J K.WW 



Fig. 176, 



Oppose each other, and hence to diminish possible sparking and 
disturbance of the field. It has the same independence of pri- 
mary and secondary voltage as the duplex motor dynamo. 
On the other hand, by reason of a common field, the period- 
icity of the currents in both windings must be the same. It 
must be remembered that a continuous current armature has a 
periodicity just as truly as an alternating armature. The cur- 
rent as generated in each is alternating, but in the former it is 
commuted before leaving the generator. Now, the frequency 
of these alternations depends directly on the number of poles 
and the revolutions per minute, being in fact the numerical 
product of the two. So if one of these composite dynamotors 
be used Vvith the continuous current winding as primary, the 



296 ELECTRIC TRANSMISSION OF POWER. 

frequency of the alternating secondary is fixed, since the 
speed of the machine cannot be changed without involving 
both primary and secondary voltages. If the alternating cur- 
rent side be used as the primary, the speed of the machine is 
fixed by the number of alternations, and whatever the voltage 
of the secondary, the frequency must be the same as that of 
the primary. Now it is a fact well known to dynairio designers, 
that continuous current dynamos generating a high frequency 
current prior to its commutation are troublesome and costly 
to build. Most continuous current dynamos have an intrinsic 
frequency of 15 to 25^^ per second. To increase these figures 
to 40^- involves some difficulty, particularly in large machines, 
while 50 to 60^' are rather hard to reach, unless in sizes of 
100 KW and below. 

Hence, in spite of the good points of the composite dyna- 
motor, it is of limited utility compared with the duplex machine 
previously described, particularly since there is a simpler way 
of doing the same work with a higher efficiency. 

This is found in the so-called rotary or synchronous converter, 
now used on a very large scale. 

This machine is nothing more than a continuous current 
dynamo fitted wdth collecting rings in addition to tjie com- 
mutator. These rings are connected to appropriate points of 
the armature winding, and supplied with alternating currents 
of the same frequency which would be generated by the arma- 
ture if the machine were used as a dynamo. The brushes 
being raised, the machine is nothing but a synchronous motor 
running without load at its normal speed. Now, when the 
brushes are put down, the alternating current simply flows 
through the armature much as if it were generated therein, is 
commuted and passes out upon the line. This commutation 
takes place under just the same general conditions as if the 
machine were used as a generator. Meanwhile a portion of 
the current supplied is passing as before, not through the 
brushes but through the winding to the collecting rings, keep- 
ing up the action as a motor. Of the total current then, a 
small part forces its way against the E. M. F. set up in the 
windings by the field, and supplies the motor function; a far 
greater part, in amoimt determined by the resistance and 



CURRENT REORGANIZERS. ' 297 

inductance of the armature, flows as if urged by this E. M. F., 
to the brushes, and supphes the generator function of the 
machine. But a single armature winding serves to drive the 
armature and to furnish a large output of commutated current. 
And this current is not simply rectified, but is of exactly the 
same character as if generated in the armature. 

Inasmuch as the armature is revolving in a magnetic field, 
the transfer of energy through the rotary converter is not in 
the last analysis a case of pure conduction and commutation. 
A part of the energy spent in the motor part of the armature 
goes into dynamical increase of output in the part which 
for the moment acts as generator. There is thus a motor 




Fig. 177. 

generator action in the same armature. Of the total energy 
delivered from the d.c. side in a monophase converter like 
Fig. 177, a little more than 40 per cent of the energy is dynam- 
ically transferred, in the polyphase forms much less, say 
12 to 24 per cent according to the number of armature taps. 
The required motor activity in the converter is thus consider- 
aVjly in excess of that required merely to spin the armature at 
synchronous speed. 

The character of the winding in a rotary converter is gen- 
erally precisely the same as in a continuous current generator, 
the only addition being two or more leads from symmetrically 
placed points in the winding to the collecting rings. These leads 



298 



ELECTRIC TRANSMISSION OF POWER. 



can be so arranged as to form a monophase system for the alter- 
nating current or, if desired, a two- or three-phase system. The 
latter forms are generally preferred, since like the correspond- 
ing synchronous motors they can be made self-starting, while 
the monophase machine has to be brought to speed by special 
and by no means simple methods. Fig. 177 shows the character 
of the armature in a simple bipolar rotary converter (mono- 
phase). Here the continuous current winding is a Gramme 
ring in 16 sections. From the brushes B, B, continuous cur- 
rent may be applied or withdrawn, while the brushes on the col- 




FlG. 173. 



lecting rings C, C, perform the same office for the alternating 
current. Such a machine may serve a variety of purposes as 
follows: 1. Continuous current dynamo. 2. Alternating cur- 
rent dynamo. 3. Continuous current motor. 4. Synchronous 
alternating motor. 5. Continuous alternating converter. 6. 
Alternating continuous converter. 

Diphase rotary converters are usually supplied with four 
collecting rings connected to form two circuits, each one join- 
ing the windings in two opposite quadrants of the armature. 
Tri phase transformers generally have three collecting rings, 




Fig. 1. 




Fig. 2. 



PLATE X. 



CURRENT REORGANIZERS. 299 

with their respective leads tapped into the windings 120° 
apart. The connections vary somewhat for different kinds of 
armature windings, but are the same in effect as those just 
indicated. One of the early practical machines of this sort 
exhibited at the Frankfort Exposition of 1891 is shown in Fig. 
178. It is of the flat ring type usual to dynamos of Schuckert 
make, and is fitted with four collecting rings outside the bear- 
ing at the comnuitator end. The rings were arranged for 
either monophase or diphase connection. The rotary converter 
thus organized attracted great attention, and was successfully 
operated in its manifold and diverse functions. It should 
be noted that if driven as a dynamo, such a machine can furnish 
continuous and alternating current simultaneously, a property 
sometimes convenient, and now not infrequently utilized. 

These rotary converters in the diphase and triphase forms 
are playing a very important part in electric railway operations 
involving considerable distances, and a large number of them 
are in highly successful use. A good idea of the modern type 
of rotary converter is shown in Fig. 2, Plate X. This is one of 
the 400 KW machines installed in 1894 to operate the electric 
railways in the city of Portland, Ore. It is designed to deliver 
continuous current at nearly 600 volts, and receives its energy 
from Oregon City, about fourteen miles away, where is in- 
stalled a triphase transmission plant. The motive power is 
derived from the great falls of the Willamette River. Current 
is generated at 6,000 volts, with a frequency of 33^ per second, 
and is given to the rotary converters at about 400 volts, 
from the secondaries of the reducing transformers. Fig. 1, 
Plate X, shows a 250 KW Westinghouse diphase machine, 
adapted for use on a 60-^ circuit and giving continuous current 
at 250 volts. An interesting feature of this machine is the 
diphase induction motor with its armature on an extension of 
the main shaft. This serves to bring the machine to speed 
without calling for the excessive current that would be required 
if the main Hues were closed upon the converter armature 
itself. The monophase form of this very interesting apparatus 
has not yet come into much practical use, not through any in- 
herent faults, but because most of the power transmission has 
so far been accomplished with diphase and triphase currents. 



300 ELECTRIC TRANSMISSION OF POWER. 

The efficiency of these machines is, as might be expected 
from their character, practically the same as ordinary con- 
tinuous current dynamos of the same output, or. rather better 
on account of the shorter average path for the current in the 
armature. In fact, so far as general properties go, they are 
dynamos. They furnish at present by far the most available 
means of deriving continuous from alternating currents, for 
they are simple, of great efficiency, and of about the same price 
as other generators of the same capacity. In point of fact, a 
well-designed polyphase rotary converter has rather better 
output and efficiency than the corresponding generator, since 
for the reason just noted the armature losses are diminished. 
Bearing this in mind, it is apparent that increasing the number 
of points at which the armature is tapped for the alternating 
current supply, thus shortening the average path to the brushes, 
will, other things being equal, lessen the armature loss. In 
practice it is found that a three-phase converter with 
three armature taps is considerably better than a monophase 
converter with two; a quarter-phase converter with four is 
somewhat better still, while a three-phase connection with 
separate phases and six taps gives even a higher output and 
efficiency. The net result is that while a monophase con- 
verter is rather inferior to the corresponding djmamo the 
two- and three-phase converters are considerably better than 
the corresponding dynamos. Quarter-phase converters are 
always connected for four collecting rings, and large three- 
phase converters not infrequently have six, to gain the advan- 
tage just mientioned. 

Efficiencies as great as 96 per cent at full load have been 
obtained from large rotary converters, with 93.6 per cent at 
half load. These figures are from a three-phase, six collect- 
ing ring converter of nearly 1,000 KW output. 

As already indicated, there is a strong tendency toward the 
use of low periodicity, 25^ to 30^- in rotary converters. This 
is partially due to the complication of the commutator in high 
frequency converters, partly to the current fashion for 
extremely low rotative speeds, and partly to lack of fi,nesse on 
the part of the average designer. That converters for a fre- 
quency as high as 60^ are entirely feasible even in capacities 



CURRENT REORGANIZERS. 301 

up to several hundred kilowatts admits of no discussion, as 
the machine put in evidence in Plate X, of which a number 
are in successful operation, plainly shows. It is undoubtedly 
easier and cheaper to build them for somewhat lower periodici- 
ties, but there seems very little reason for going so low as is the 
current custom, and it tends needlessly to multiply special 
types of apparatus. 

And yet the simplicity of the rotary converter is attained 
at the cost of certain practical inconveniences that cannot 
lightly be passed by. Their source is the employment of a 
single field and armature winding for all the purposes of the 
apparatus. The results are, first, complete interdependence 
of the alternating and continuous voltages, and, second, con- 
sequent difficulties of regulation that are occasionally very 
troublesome. 

The immediate result of a single winding is that there is an 
approximately fixed ratio between the alternating and the con- 
tinuous voltage. The former is always the less, and, while 
varied by changes in the number of phases determined by the 
connections, is approximately the alternating voltage that 
would be yielded by the machine driven as a generator. This 
is, for monophase or diphase connections, about seven-tenths 
of the continuous current voltage, and for three-phase connec- 
tions about six-tenths. The proportions would approximate to 

1 V3 

I- and ^ respectively, if the alternating E. M. F.'s were 

V2 2 V2 

sine waves, which they never are when derived from an ordi- 
nary continuous current armature. In service the real pro- 
portions may, and generalh^ do, vary by several per cent, 
according to the excitation. In a particular two-phase case 
the actual ratio was .68, and in a three-phase case .65. If, 
therefore, a rotary converter be used for supplying continuous 
current, the applied alternating current must be of lower pres- 
suro than the derived continuous, in about the proportion 
above noted. This compels the use of reducing transformers 
in every case of power transmission involving this apparatus. 
Further, any cause that affects the alternating pressure affects 
the continuous as well. Line loss, inductance, resonance effects, 
as well as changes at the generators, all influence the vol- 



302 ELECTRIC TRANSMISSION OF POWER. 

tage at the continuous current end of the rotary transformer. 
Nor can this voltage be freely altered by changing the field 
strength, since, as we have already seen, this may profoundly 
change the inductance of the alternating circuit, which is for 
many reasons undesirable. The field windings of rotaries are 
either shunt from the d.c. side or compound. The former 
winding gives much the steadier power factor and, hence, 
is rather desirable for close regulation of a steady load, while 
the latter is advantageously used for railway loads and the hke. 
The best results are obtained by carefully adjusting the gen- 
erator, line, and rotary converters to work together. Other- 
wise there is likely to be trouble in regulation. 

For these reasons in cases where close regulation is neces- 
sary, as for incandescent fighting, preference has frequently 
been given to the motor generator with double field and arma- 
ture, as in the large Budapest system installed by Schuckert & 
Co., who were among the pioneers in developing the rotary 
converter. In this case the transmission is at- 2,000 volts 
diphase, at which pressure current is delivered to the motor 
end of the motor generators placed in substations at conve- 
nient points. In such a plant the increased cost of the duplex 
machines is not so great as might be supposed, for reducing 
transformers are needless, and the output of both generators 
and motors can be forced to the utmost fimit of efficient oper- 
ation, without fear of injuring the regulation, which is reduced 
to the easy problem of accurately compounding a continuous 
current generator. The net efficiency of the Budapest trans- 
formation is said to be 85 per cent. Some recent experiments 
on the relative efficiency and cost of motor generators and 
rotary converters are as follows : The sets compared were of 
200 KW capacity for changing triphase current from the 
Niagara circuits at 11,000 volts, 25^ into continuous current 
at 120 to 150 volts. The efficiencies given are net, including 
the necessary provisions for obtaining a variation of 25 per 
cent in the finally resulting voltage: 





Motor- 
Generator. 


Transformers 
and Rotaries. 


Difference. 


Full load 


87.40 


89.87 


2.47% 


f load 


85.54 


88.70 


3.16% 


^load 


81.42 


84.90 


3.48% 



CURRENT REORGANIZERS. 303 

The extra apparatus required with the rotaries brought the 
two methods to substantially the same cost, but for Hghting 
work the motor generators gave the better results. 

From the foregoing it is sufficiently evident that every case of 
current reorganization cannot be successfully met by the same 
apparatus. For certain small work the rotating commutator 
seems to be fairly well suited, and for occasional purposes it 
is somewhat cheaper and more efficient than any of its rivals. 
Next in point of efficiency and cheapness comes the rotary 
converter, infinitely better for heavy work than any commutat- 
ing device, and finding very extensive application to electric 
railway work. Finally, for work requiring very close regulation, 
the motor generator is specially well suited, closer to the rotary 
transformer in cost and efficiency than would be supposed off- 
hand, and unique in the complete independence of its working 
circuits. 

Practice in this line of operations has not yet settled into 
fixed directions, and is not likely so to do just at present. 
Each plant nmst therefore be considered by itself and treated 
symtomatically. 

American usage is at present tending strongly toward the 
rotary converter, on account of its ready adaptation to railway 
service, but, in view of the work that has been done on alternat- 
ing motors for such service, it is an open question how far 
current reorganization will be generally necessary in the future, 
although just now it is of very great practical importance. 

As the price of copper rises, the use of current reorganizers 
becomes more and more important in railw^ay work, and for 
this particular use the rotary converter is generally chosen. 

There should be mentioned here some curious and valuable 
devices for obtaining rectified alternating currents, based 
upon the phenomena of polarization. 

Obviously, if one could find a conductor which would let pass 
currents in one direction, and block those in the other, the 
result of putting it in an alternating circuit would be that all 
the current impulses in one direction would be suppressed, so 
that the resulting current would be a series of separated half- 
waves of the same polarity. It would be as if in Fig. 166 all 
the half-waves above the base line were erased. Now such a 



304 ELECTRIC TRANSMISSION OF POWER. 

conductor is actually obtainable in certain electrolytic cells in 
which a counter electromotive force or severe polarization 
resistance impedes current flowing in a particular direction. 
Under favorable circumstances the selective action is quite 
complete, so that the alternating current becomes miidirec- 
tional. Fig. 179 shows the current curve for a complete 
cycle as modified by electrolytic rectification. The positive 
half of the wave is practically wiped out of existence. The 
efficiency of these electrolytic devices as regards the energy 
rectified is quite low, and most of the apparatus constructed 
has been upon a very small scale, but there are certain purposes, 
like energizing induction coils, for which it may occasionally 
be of service. It is given place here more on account of its 




Fig. 179. 

general interest than for any practical value. It works best, 
like most other rectifying devices, at low frequencies. 

The latest and in some respects the most interesting device 
for obtaining continuous currents from an alternating source, 
is the vapor, or mercury arc converter. Its action depends 
on the mechanism of current flow in the electric arc. As is 
well kno"v\Ti, the current is carried across the space between the 
terminals of an electric arc by a blast of vapor streaming from 
the negative to the positive electrode. An arc cannot start 
until this stream has been established, for which reason arcs 
are generally started by touching the electrodes momentarily 
together. For the same reason on a low frequency alternating 
circuit, or generally unless a considerable mass of conducting 
vapor lingers between the poles, the arc readily goes out, since, 
granted that the arc is struck at all, the negative stream dies 
with the pulse of current that produced it, and the following 
alternation can only get through by starting a new stream 
from the other electrode as negative. 

Now, the arc formed about a mercury negative pole in vacuo 



CURRENT REORGANIZERS. 



305 



has this remarkable property, that, while once started the 
stream can be maintained by a few volts, it takes many thou- 
sand volts to initiate or to reestablish the stream over any 
material gap. Hence, if the stream is once started it can be 
kept in action continuously by a rather low voltage current, 
but can be reversed only by an enormous E. M. F. in the 
opposite direction. 

If, however, the original negative stream can be kept going 



Positive Eloctrodes. 




it will transmit freely current impulses in the original direc- 
tion while reverse impulses will lack the potential required to 
reverse the stream. Upon this property of the mercury arc 
the vapor converter is based, and the essential feature of its 
operation is the preservation of the negative stream by send- 
ing overlapping impulses, so that once started the original 
stream shall not die out. The extremely ingenious method 
of doing this is shown in Fig. 180. Here A is an exhausted 
bulb 8 or 10 inches in diameter, containing two positive elec- 
trodes side by side, and a mercury negative electrode D. 



306 



ELECTRIC TRANSMISSION OF POWER. 



At 5 is a fairly stiff reactance. The two positives are connected 
to the terminals of an auto-converter C and its middle point 
is connected to B through the proposed d. c. circuit. 

The apparatus is started by tipping the bulb until a supple- 
mentary mercury positive touches the negative and as the 
bulb is tipped back the negative stream starts. 

Let us say that the current let through is via the right hand 
electrode. Owing to the reactance B the current, lagging, 
persists until the E. M. F. rising in the left hand connections 




Pig. 181. 



has had time to start via the same negative stream, a current 
through the other positive electrode. Positive electrodes 
virtualty in the same negative vapor blast can thus exchange 
work freely, provided the blast be not interrupted. 

The two sides of the circuits thus keep up the interchange, 
working alternately, but utilizing as will be seen from the 
consecutive directions of flow, both sets of alternations. By 
this same cause the effective E. M. F. of the rectified current 
is something less than half the nominal a. c. voltage applied 
to the apparatus as a whole. By a stroboscopic examination 
the sequence of the operations can be very beautifully seen. 



CURRENT REORGANIZERS. 307 

Two-phase or three-phase currents can be made operative in 
a very similar manner so that the process is a general one. 
It can be made operative at any commercial frequency. 

Fig. 181 shows the constant current form of the same device. 
The letters have the same significance although the electrode 
tube is of different shape, and the coil C is here the secondary 
of a constant current transformer. The resulting current from 
the vapor converter is evidently not uniform but somewhat 
pulsatory as if received from a dynamo having very few seg- 
ments in the commutator. 

Fig. 182 from an oscillograph record * of the current form 




Fig. 182. 

derived from the constant current converter Uke Fig. 181, 
shows the facts in the case admirably. 

The efficiency of such apparatus is high. There is a small 
back E. M. F. of about 15 volts to overcome, the ohmic and 
hysteretic loss in the transformer and reactance, and some 
heating of the converter tube. The higher the voltage applied 
to the tube the less current for a given energy and the better 
the efficiency, and the voltage may be anything that will not 
strike a reverse arc in the" tube. At current of a few amperes 
the working a. c. voltage may even be 25,000 volts. At mod- 
erate voltages the back E, M. F. is more important and the 
current rises for the same energy so as to sooner reach the 
heat endurance of the tube. 

The constant potential form is now commercially avail- 
able in moderate capacities, say up to 25 or 30 KW at 115 to 
120 volts, the efficiency being about 75 to 80 per cent. These 
converters are designed for charging storage batteries and 
* Steinmetz, tr. A. I. E. E. June, 1905. 



808 



ELECTRIC TRANSMISSION OF POWER. 



similar light work. The constant current form is beginning 
to be used for arc lights, giving d. c. arcs off an a. c. circuit, 
using d. c. voltages up to 4,000 or 5,000 volts. The efficiency 
of such sets is probably between 80 and 90 per cent, and the 
power factor is reported to be .90 or better. 

The apparatus is very beautiful in principle, and has thus 
far developed no serious operative defects. Its life is somewhat 
uncertain and a good deal of experimenting is still needed to 
bring it into standard form, but it is altogether very promising. 
Whether it is to be available for large powers remains to be 
seen, but it is certain to find a wide commercial use so soon 
as it has been far enough standardized to enable the price to be 
brought down to a manufacturing basis. At present the 
figures are too near those charged for motor generators to 
encourage any widespread enthusiasm. 

Of late two very interesting converting devices have come into 
use abroad. The first is the " cascade converter " of which the 



s 




/oq' qW 



1 Lroij-oWC"^ 




Fig. 183. Circuits of Cascade Convekter. 

circuits are shown in diagram in Fig. 183. The machine consists 
virtually of an induction motor directly coupled to a rotary con- 
verter, each being of about half the total rated output. In the 
figure S is the stator winding of the motor end, E the rotor wind- 
ing, R^ the starting resistance. On the generator end G is the 
armature winding, K the commutator, F the shunt field, and L 
the load. Both ends have ordinarily the same number of poles. 
The rotor and the converter armature being in series the normal 
counter E. M. F. is reached at half synchronous speed, and com- 
mutation takes place at half the initial frequency, which is a 
great advantage. Similarly half the energy is delivered to the 
output end of the machine by the rotor as frequency changing 
transformer, and the rest by torque on the shaft. These cascade 
converters have admirable operative qualities, and have come 



CURRENT RE ORGANIZERS. 



809 



into use with excellent results in several foreign railway plants 
in preference to ordinary motor generators or rotary converters. 

The second device for the same purpose is known as the ^^ per- 
mutator." As at present made it is a vertical axis machine 




Fig.- 184. A^ERTicAL, Section of Permdtator. 

shown in section in Fig. 184. It is, in effect, much like an 
induction motor, with the rotor fixed and provided with a com- 
mutator around which the brushes are moved in synchronism. 
The rotary field is substituted for the physical rotation of an 
armature while the brushes follow it, synchronously driven by a 
motor armature requiring but a modest input. The efficiency 
of these permutators is said to be very high, materially greater 
than that of an ordinary rotary converter, and the space required 
and the weights involved are very moderate. The preliminary 
reports from the permutator are good, but it has not yet come 
into sufficient use to give a fair basis for a final judgment. The 
conditions for commutation do not look altogether favorable, and 
rotating brushes are somewhat objectionable from a practical 
standpoint, so that one would expect the machine to be of special 
rather than of general usefulness. 



CHAPTER VIII. 

ENGINES AND BOILERS. 

Mechanisms that constitute the Hnk between natural sources 
of energy and mechanical power are called prime movers. So 
far as the electrical transmission of energy is concerned, but 
two classes of prime movers, steam engines and water-wheels, 
have to be seriously considered. All others sink into insigni- 
ficance or are limited to special and rarely-occurring cases. 
When power is transmitted electrically over considerable dis- 
tances the prime mover is usually a water-wheel, since, as yet, 
the transmission of power from coal fields has been hardly 
more than begun, although when long electrical lines be- 
come somewhat more familiar, coal may become a frequent 
source of energy. Where the distribution of power from a 
central point is to be accomplished, the prime mover is fre- 
quently a steam engine. 

The general principle of the steam engine may be fairly 
supposed to be somewhat familiar to the reader, but the con- 
ditions of economy are not always so clearly understood. The 
source of power in an engine is the pressure of the steam, 
which must be utilized as fully as possible to get anything like 
efficient working. Since the pressure is in direct proportion 
to the temperature in any gas, the proportion of the total pres- 
sure which can be used depends on the original temperature 
at which its use is begun, and the temperature at which one 
ceases to use it and rejects it together with all the energy it 
then possesses. These temperatures are not to be reckoned 
from the ordinary zero of a thermometer, but from the so- 
called absolute zero. This is that point from which, if the 
temperature of a gas be reckoned, its pressure will be directly 
proportional to the temperature. It is 461° below zero, 
Fahrenheit, that is, 493° below the melting point of ice. It is 
determined by the consideration that any gas at this melting 
point loses 4^5 of its pressure for a change in temperature of 

310 



ENGINES AND BOILERS. , Bll 

one degree, hence, if it could be cooled down 493°, would lose 
its pressure and would have given up all of its energy. Count- 
ing from this absolute zero, then, one can utilize that part of 
the whole energy of a gas which lies between the temperature 
at which the gas begins to work and that at which it ceases to 
do work. In other words the efficiency of any engine operated 
by gaseous pressure is: 



T 



in which T^ is the absolute temperature of the gas when 
it begins to do work in the engine, and T2 the absolute temper- 
ature at which its work ends. In practice, T-^ is the tem- 
perature of the steam when it enters the cylinder, and T^ 
the temperature of exhaust or condensation. Steam permits 
the use of but a limited range of temperature on account 
of the temperature at which it liquefies, and bothers us by 
condensing as it expands, even in the cylinder. It must be 
remembered that while we are limited by our possible range of 
temperature to a low total efficiency in any heat engine, of the 
energy that can possibly be obtained within this limitation, 
a very good proportion is recovered in the best modern engines 
— from one-half to three-fourths. The remainder is lost in 
various Avays, largely through radiation of heat and cylinder 
condensation. Besides these thermal losses a portion of the 
energy utilized is wasted in friction of the mechanism. 

From these considerations w^e may derive the following 
general principles of engine efficiency: 

I. The steam should be admitted at the highest pressure 
feasible and exhausted at the lowest pressure possible. 

This indicates that high boiler pressure should be used, and 
that it is better to condense the steam than to expel it into the 
air, as by condensing most of the atmospheric pressure can 
be added to the working range of pressure in the engine. In 
the next place it is evident that the steam should be sent into 
the engine at full boiler pressure, and finally condensed after 
expanding and yielding up its pressure as completely as 
possiV;le. 

II. Waste of heat in the engine should be stopped as far 



312 ELECTRIC TRANSMISSION OF POWER. 

as possible. This means checking losses from the cylinder 
by radiation and conduction, and internal loss from cylinder 
condensation. The first principle laid down has for its ob- 
ject the increase of the possible efficiency, while this second 
principle bears on the securing of as large a proportion as 
possible of this possible efficiency. It requires the preven- 
tion of escape of heat externally by protecting the cylinder, 
and incidentally shows the advantage of high pressure and high 
piston speed in securing as nmch work as possible without in- 
creasing the size of the working parts, and hence their chance 
for radiation. On the other hand, it indicates the danger of 
working with too great a range of temperature in the cylinder 
thus producing cylinder condensation. 

III. The work of the engine should be the maximum practi- 
cable for its dimensions and use. This secures high mechan- 
ical efficiency as the previous principles secure high thermal 
efficiency. To fulfill this condition high steam pressure and 
high piston speed are necessary, and the latter usually means 
also rather high rotative speed. The importance, too, of fine 
workmanship in the moving parts is evident. 

It will be realized that some of the conditions just pointed 
out are mutually incompatible to a certain extent. Every- 
thing points, however, to the great desirability of a condens- 
ing engine, worked with a high initial steam pressure and 
great piston speed. The tendency of the best modern prac- 
tice is all in this direction, and the efficiency of engines is con- 
stantly improving. The greatest advances of the past decade 
or two have been in the introduction of compound engines. 
The principle here involved is the lessening of thermal 
losses in the cylinder by avoiding extremes of temperature 
between the initial and the final temperature of the steam ex- 
panded into it. Compound engines simply divide the expan- 
sion of the steam between two or more cyhnders, so that the 
temperature range in each is limited, without limiting the 
total amount of expansion. 

Following the same line of improvement, triple and quad- 
ruple expansion engines are becoming rather common, although 
the value of the last mentioned is somewhat problematical at 
present. 



ENGINES AND BOILERS. 313 

For practical purposes steam engines may be classified in 
terms of their properties, somewhat as follows : 

First, there is the broad distinction between condensing and 
non-condensing engines. The former condense the exhausted 
steam and gain thereby a large proportion of the atmospheric 
pressure against which the latter class is obliged to do work in 
exhausting the steam. Where economy of operation is se- 
riously considered, the non-condensing engine has no place, if 
water for condensation is obtainable. 

Each of these classes falls naturally into subclasses, depend- 
ing on the number of steps into which the expansion is 
divided — simple, compound, triple expansion, etc. Of these 
the first may now and then be desirable, where the size is small 
and coal very cheap, but for the general distribution of energy 
the last two are more generally useful. Furthermore, each of 
the subclasses mentioned may be divided into two genera, 
depending on the nature of the valve motions that control the 
admission and rejection of the steam. To follow out the first 
principle of economy laid down, the steam must be admitted 
at a uniform pressure as near that of the boiler as possible, the 
admission should be stopped short after entrance of enough 
steam for the work of the stroke, the steam allowed to expand 
the required amount, and then rejected completely at the lowest 
possible pressure. The admission valves should therefore 
open wide and very rapidly, let in the steam for such part of 
the stroke as is necessary, and then as promptly close. The 
exhaust valves should open quickly and wide when the expan- 
sion is complete, and stay open until nearly the end of the 
stroke, closing just soon enough to cushion the piston at the 
end of its stroke. In proportion to the completeness with 
which these conditions are met, the use of the steam will be 
economical or wasteful. The two genera of engines referred to 
are those in which the motions of the aduiission and exhaust 
valves are independent of each other or dependent. Fig. 185 
shows in section the cylinder and valves of an independent 
valve engine, Corliss type. The arrows show the flow of the 
steam. The admission valve on the head end of the cyhnder 
has just been opened, as also has the exhaust valve on the 
crank end. 



314 



ELECTRIC TRANSMISSION OF POWDER. 



The essential point of the mechanism is that the admission 
valves open and close at whatever time is determined by the 
action of the governor without in the least affecting the work- 
ing of the exhaust valves. In the Corliss valve gear the 
steam valves are closed by gravity, or by a vacuum pot, and 
are opened by catches moved by an eccentric rod, and released 
at a point determined by the governor, which thus varies the 
point of cut-off according to the load. Ordinarily the admis- 
sion of steam is thus cut off in a simple engine at full load 
after the piston has traversed from one-fifth to one-quarter of 
its stroke, according to the pressure of the steam. If the cut- 
off is too late in the stroke, there is not sufficient expansion of 




^'■-' >-'-'-■-" ' ■: 



^yyyyyyyyyyyyyyyyy.yyyyyy^: ^ 



^^^^^^^^^^^^^^^^^^^^1 



\^^^^^^^^^^^^^^^^^^^^^^^^^^^ 



^^^^^^^^ 



k^yy^^^yyyyyyyyyyyyyyyy^A 




Fig. 185. 



the steam; if too early the steam is partially condensed by too 
great expansion. For every initial pressure of steam there is a 
particular degree of expansion which gives the best results in a 
given engine. 

Fig. 186 shows the valve motion of one of the best of the 
dependent valve genus. Steam is just being admitted at the 
head end both around the shoulder of the hollow piston valve 
and through the ports at the other end of the valve via the 
interior space. At the crank end the exhaust port has just 
been fully opened. It will be seen that any change in the 
conditions of admission also involves a change in the condi- 
tions of exhaust, and although some variation may take place 
in the latter without serious result on the economy, simplicity 



ENGINES AND BOILERS. 



315 



in the valve gear has been gained at a certain sacrifice of 
efficiency in using the steam. Both independent and depen- 
dent valve engines have many species differing widely in 
mechanism, but retaining the same fundamental difference. 
Of the two genera, the independent valve engine has the 
material advantage in efficiency, and under similar conditions 
of pressure, capacity, and piston speed consumes from 10 to 20 
per cent less steam for the same effective power. It there- 




FlG. 186. 



fore is generally employed, in spite of somewhat greater first 
cost, for all large work, often in the compound or triple ex- 
pansion form. Except in small powers, or for exceptionally 
high speed, the dependent valve engine has few advantages, 
and in the generation of power on a large scale, such as for 
the most part concerns us in electrical transmission work, it 
hardly has an important place. 

It must not be supposed that between the various sorts of 
engines mentioned there are hard and fast lines. In the 
economical use of steam a very large non-condensing engine 



316 ELECTRIC TRANSMISSION OF POWER. 

may surpass a smaller condensing one, or a fast running 
dependent valve engine, a very slow running one with inde- 
pendent valves. Broadly, however, we may lay down the 
following propositions concerning engines of similar capacity: 

I. Condensing engines will always furnish power more 
economically than non-condensing ones. This is particularly 
true at less than full load, since the loss of the atmospheric 
pressure may be taken as a constant source of inefficiency, 
which, like mechanical friction, is very serious at low loads. 
For example, a triple expansion engine working at one-quarter 
load in indicated HP, will be likely to have its consumption of 
steam per IHP, increased from 15 to 25 per cent above the con- 
sumption per IHP at full load; while worked non-condensing, 
the increase would be from 50 to 100 per cent. Hence, for 
electrical working where light loads are frequent, condensing 
engines are an enormous advantage. With simple or compound 
engines the same general rule holds good as for triple-expan- 
sion engines, with the additional point that light loads affect 
their economy even more, when worked non-condensing. It 
must be borne in mind that if any engine is to do its best under 
varying loads, its valve gear and working pressure must be 
arranged with this in mind, else the advantage of high expan- 
sion and condensing may be thrown away. It is frequently 
said that triple expansion engines do not give good results in 
electric railway work. When this is the case there has been 
improper adjustment of engine to load. 

II. Among engines having the same class of valve gear, 
compound engines give better economy than simple ones, and 
triple expansion better than compound. This is true irre- 
spective of the nature of the load, supposing each engine to be 
suitably adjusted to the work it has to do. In rare cases, owing 
to exceedingly cheap fuel and short working hours, it may hap- 
pen that the advantage of a triple expansion engine over a 
compound in economy of coal may be more than offset by 
increased interest on investment, but at the present cost of 
engines and boilers, this could not well occur unless in the 
case of burning culm or poor coal obtained at a nominal 
price. 

III. As regards speed of engines, there is always advantage 



ENGINES AND BOILERS. 317 

in high piston speed both as respects first cost and mechanical 
efficiency. So far as the economical use of steam goes, speed 
makes little difference save as it sometimes involves a change 
in the valve gear. Most high-speed engines have valve gear 
of the dependent sort, which puts them at a disadvantage 
except in so far as lessened cylinder condensation and friction 
may offset the losses due to less efficient distribution of the 
steam. But the best dependent valve engine is uniformly less 
economical than the best independent, valve engine of the 
same class and subclass. Even the lessened friction of the 
small high-speed pistons does not offset this difference in 
intrinsic economy. 

As regards actual economy in the steam consumption, the 
size of engine has a powerful though somewhat indeterminate 
inffiience. Even at full load, simple non-condensing dependent 
valve engines of moderate size require from 30 to 40 lbs. 
of steam per indicated horse-power hour. Only in very large 
engines, such as locomotives, and specially fast running engines 
such as the Willans, does the steam consumption of these 
dependent valve engines fall below 30 lbs., and not very often 
even in these cases. Worked condensing the same machines 
use from 20 lbs., in exceedingly favorable cases, to 25 or 30 lbs. 
more commonly. 

Independent valve engines, simple and non-condensing, will 
give the indicated HPH on 25 to 30 lbs. of steam, occasion- 
ally on as little as 22 to 23 lbs. With the advantage of con- 
densation these figures may be reduced to say 18 to 25 lbs., 
the former figure being somewhat exceptional and probably 
very rarely attained in practice. 

Passing now to compound non-condensing engines, the effect 
of compounding on efficiency is about the same as that of con- 
densing. Ordinary dependent valve engines of compound 
construction require from 20 to 25 or 30 lbs. of steam per 
IHP hour. The former result is very exceptional, and seldom 
or never reached in practice, while the last mentioned would 
be considered rather high. Independent valve compound 
engines are so seldom worked non- condensing, that the data of 
their performance are rather meagre; 18 to 25 lbs. of steam 
is about the usual amount, however. 



318 ELECTRIC TRANSMISSION OF POWER. 

When condensation is employed, on the other hand, the 
dependent valve engines are in rather infrequent use. When 
the need for economy is so felt as to lead to the use of com- 
pound engines, it also leads to the use of economical valve 
gear. The steam consumption of dependent valve compound 
condensing engines is quite well known, however, and is 
usually from 16 lo 24 lbs. per IHP hour. The first mentioned 
figure is rarely reached, and only in special types of engines. 

Plenty of tests on compound condensing engines with inde- 
pendent valves are available; 14 to 20 lbs. of steam covers 
the majority of results. Occasional tests rim down to and 
even below 12 and as high as 22 lbs. 

It is noticeable that in compound engines the difference 
between dependent and independent valve gear is less than 
with simple engines. This is due to a variety of causes. The 
larger range of expansion used in compound engines tends to 
lessen the deleterious effects of moderate variations in the 
distribution of the steam, and besides, the valve gear of com- 
pound engines is not infrequently composite, the high-pres- 
sure cylinder having independent valves and the low-pressure 
cylinder dependent ones. 

The same arrangement is often used in triple expansion 
engines, so that, in conjunction with the condition before 
mentioned, it is usually true that the economy of dependent 
valve triple expansion engines is much nearer that of indepen- 
dent valve ones than would be at first supposed. Without 
condensing, a dependent valve triple expansion engine may be 
expected to require from 19 to 27 lbs. of steam per IHP hour. 
With condensation such engines perform much better, the steam 
consumption being reduced to 14 to 20 lbs. 

Nearly all triple expansion engines, however, are built with 
independent valves, at least in part, the intention being to 
secure the most economical performance possible. Under 
favorable conditions their steam consumption runs as low as 
12 lbs. per IHP hour, and seldom rises above 18 lbs. In a 
few exceptional cases the record has been reduced below 
12 lbs., but such results cannot often be expected. Any- 
thing under 13 lbs. of steam per HP is good practice for 
running conditions. 



ENGINES AND BOILERS. 



319 



All the figures given refer in the main to good sized engines 
of at least 200 HP and over, operated at full load and at 
favorable ratios of expansion. It must be clearly understood 
that there is for each steam pressure a particular ratio of 
expansion which will give the most economical result — less ex- 
pansion than this rejects the steam at too high a temperature; 
more, causes loss by condensation, etc. Compound and triple 
expansion engines permit greater expansion of the steam with- 
out loss of economy, hence allow higher steam pressure and a 
greater temperature range — hence higher thermal efficiency. 
Good practice indicates that for simple engines the boiler 
pressure should be not less than 90 to 100 lbs. per square 
inch, for compound engines not less than 120 to 150, and for 
triple expansion engines not less than 140 to 150, and thence 
up to 175 or 200 lbs. 

We may gather the facts regarding steam consumption into 
tabular form somewhat as follows : 



Kind of Engine. 


steam per IHP. 
General Range. 


steam per IHP. 
Working Average. 


Simple, non-condensing dep. v 


30-40 
25-30 
20-30 
18-25 
20-28 
18-25 
16-24 
14-20 
14-20 
12-18 
12-14 


33 


Simple, non-condensing indep. v 

Simple, condensing dep. v 


28 
25 


Simple, condensing indep. v 


21 


Compound, non-condensing, dep. v 

Compound, non-condensing indep. v 

Compound, condensing dep. v 


24 

22 
20 


Compound, condensing indep. v 


17 


Triple condensino" dep v . . 


17 


Triple, condensing indep v .... 


14 


Triple large, condensing indep. v 


13 



The engines considered are supposed to be of good size — 
say 200 to 500 HP, and to be worked steadily at or near full 
load. The figures given as working average are such as may 
be safely counted on with good engines, kept in the best 
working condition, and operated at at least the boiler pres- 
sures indicated. The steam is supposed to be practically dry 
and the piping so protected as to lose little by condensation. 
These results are such as may regularly be obtained in prac- 
tice, and indeed it is not uncommon to find them excelled. 
Compound condensing engines of large size not infrequently 



320 ELECTRIC TRANSMISSION OF POWER. 

work down to 13 lbs. of steam, and triple expansion con- 
densing engines down to 12 lbs., which result will be guar- 
anteed by most responsible builders. 

Unfortunately, engines employed for electrical work are com- 
paratively seldom kept at uniform full load. Furthermore, 
they are subject to all sorts of variations of load. In electric 
railway service there are sudden changes from light loads to 
very heavy ones, while in electric lighting there is generally a 
gradual increase to the maximum load, which continues an 
hour or two, followed by a rather gradual decrease. These 
variations affect the economy of the engines unfavorably — at 
certain loads there is not enough expansion, at others decidedly 
too much. The variations in economy are largely controlled 
by the proportioning of the engine to its work. To say that 
an engine is of 500 HP means little unless the statement be 
coupled with a definite explanation of the circumstances. If 
that output is obtained by admitting steam for half the stroke, 
the engine will work at 500 IIP very uneconomically, sup- 
posing a simple engine to be under consideration. Its point 
of maximum economy may be perhaps 300 HP. On the other 
hand, 500 HP may be given when cutting off the steam at one- 
fifth stroke. In this case the engine will be working near its 
point of maximum economy, and at 300 HP will require much 
more steam per IHP. It could give probably 600 to 700 HP 
at a longer cut-off, and is really a much more powerful engine 
than the first. For uniformity it is better to rate an engine at 
the HP of maximum economy, whatever the real load may be. 
The relation of load to economy is well shown in the curves of 
Fig. 187. 

Curves 1, 2, 4, and 5, are of engines so rated as to have 
their maximum economy near full load. Curve 3, on the 
other hand, is from an engine intended to give its highest 
economy at about three-quarters load. For very variable 
output this is the preferable arrangement, while for large 
central station work, when the number of units is large enough 
to permit loading fully all that are running at any one time, 
it is better to have each unit give its very best economy near 
full load and to vary the number of units according to the re- 
quirements of total load. 



ENGINES AND BOILERS. 



321 



For electric railway service under ordinary conditions, it is 
best to employ an engine which at full load is worked to a 
high capacity, and hence somewhat uneconomically, while at 
lesser loads, which more nearly correspond with the average 
conditions, its economy will be at a maximum. For electric 
lighting service it is preferable to have the point of maximum 




PER CENT LOAD I.H.P» 
Fig. 187. 



economy fall more nearly at full load. For power service, 
which is on the one hand more uniform than railway service, and 
less uniform than electric lighting work, it is probably best to 
employ an engine having characteristics between those just 
mentioned. In every case attention must be paid to the 
character of the load as regards average amount and con- 
stancy in the choice of an appropriate engine for the work. 



322 



ELECTRIC TRANSMISSION OF POWER. 



In cases where the variations of load are Hkely to be very 
sudden, great mechanical strength of all the moving parts is 
absolutely necessary, and an attempt should be made in plan- 
ning the power station to arrange the engine for its best econ- 
omy at average load as nearly as this can be predicted. 

With care in planning an electric power station the engines 
can be made to give an exceedingly good performance, much 







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0.25 0.60 0.75 1.0 1.35 

PROPORTION THAT ACTUAL LOAD BEARS TO RATED POWER 

Fig. 188. 



1.50 



better than was considered possible a few years ago. Fig. 
188 shows a set of curves from the experiments of Prof. R. C. 
Carpenter giving the performance of engines of different kinds 
over a wide range of loads, from mere friction load up to 50 
per cent overload. The results are in pounds of water 
evaporated per indicated HPH. The immense advantage to 
be gained by using compound and triple expansion condensing 



ENGINES AND BOILERS. 323 

engines appears plainly from the curves. Another conspicu- 
ous fact is the great economy attained by such engines over a 
wide range of load. It is a common fallacy to suppose that 
while compound or triple expansion condensing engines are all 
well enough at steady load, simple engines have the advantage 
if the load varies over a wide range. The facts in the case as 
shown in Fig. 188 are exactly the reverse: not only do the high 
expansion engines have the advantage of the simple engines 
at their rated loads, but at all loads, and particularly light ones. 
And their advantage is so great that under any ordinary cir- 
cumstances the use of a simple or a non-condensing engine for 
power generation is wilful waste of money. If the saving in 
first cost were great the mistake might be excusable, but the 
greater amount of steam required for running simple engines 
means larger boiler capacity, which nearly offsets the lower 
cost of engine. For example, a glance at Fig. 188 shows that 
a triple expansion condensing engine requires only half the 
boiler capacity demanded by a non-condensing automatic engine 
for the same output. In other words, if the former requires 
500 HP in boilers, the latter will need 1,000 HP in boilers for 
exactly the same service. And the same holds true of the 
capacity of the stack, feed-pumps, steam-piping, water-piping, 
and, to a certain extent, even of the building, so that it is 
almost always poor economy to buy a cheap type of engine. 

The greatest improvement in economy made in recent years 
has been the introduction of superheating which American 
engineers have been somewhat slow in adopting. This is simply 
the heating of the steam as such on its way to the engine. 
The steam prior to use is passed through a special reheater, 
frequently with an independent furnace, and given additional 
heat energy, the working temperature being thus raised 
sometimes to 600° or 700° F. This largely increases the 
range of working temperature possible to the engine, and hence 
the efficiency, at a relatively small expense for extra fuel. 

The very high temperature of the steam compels extra 
precautions in the lubrication, and for some years this lubri- 
cation bug-a-boo stood in the way of substantial progress. 
At present it is entirely practicable to lubricate the cylinders 
successfully even up to the figures mentioned above, and 



324 



ELECTRIC TRANSMISSION OF POWER. 



it is being done abroad though the prejudice in this country 
still persists. The results are startling, the steam consumption 
under test having repeatedly run down near and even below 
10 lbs. of steam per IHP hour in compound condensing engines. 
The result of a recent test of a 21 x 36 X 36-inch mill engine 
are given in Fig. 189 and represent the highest efficiency yet 
attained. It will be noted that under the test conditions with 
steam superheated to 720° - 750° F. the steam consumption 
increased for the heavy loads just as it rises for overloads in 
the curves of Fig. 188. The fact in each instance merely 
implies that a certain amount of expansion corresponds to 



9.5 












































X 


^ 
































8.5 


















































fi 



























300 
LOAD 

Fig. 189'. 



maximum economy, and less than this amoimt injures econ- 
omy although it increases the possible output. 

Fig. 189 is merely an extreme instance of the general principle, 
due to starting the expansion at a relatively very high temper- 
ature. There is no doubt whatever of the practicability of 
reducing steam expenditure 20 to 30 per cent below that 
found in the best current practice by an amount of super- 
heating applicable without any considerable difficulty. Super- 
heaters have already been introduced here as auxiliaries to 
steam turbines with pretty good effect, but have not yet come 
into more than occasional use for general purposes, and even 
so are very rarely worked for what they are really worth 

Large gas engines are beginning to come into use as prime 
movers for electrical purposes, and one such plant of 12,000 
KW capacity is just being installed in San Francisco. The 
gas engine in large sizes shows very great thermal efficiency, 
giving the brake HP hour on the thermal equivalent of 1 lb. 



ENGINES AND BOILERS. 825 

of coal or even less, and is to-day becoming a formidable com- 
petitor of steam engines for many purposes. Working as it 
does from a very high initial temperature, its theoretical claim 
to efficiency is valid enough, and the difficulties of lubrication 
at the temperature involved have proved less serious than 
was first supposed. The main trouble is the fact that ordi- 
narily only every fourth stroke is a working stroke, so that for 
a given number of impulses per revolution of the fly-wheel the 
gas engine becomes far more heavy and complex than the 
steam engine. Nevertheless, the gain in fuel economy is so 
valuable that the incentive to use gas engines is great. They 
are usually worked in the large sizes with natural or ^' producer'^ 
gas, sometimes with gas from the blast furnaces of the steel 
industry, in other words with cheap gas unsuited for illumi- 
nating purposes, and have the merit of being very quickly 
brought into action when required. Difficulties of governing, 
once serious, have now been in great measure eliminated. 

Many blunders are made by being too hasty in buying 
engines for electric service, and not sufficiently studying the 
problem. For uniform loads the selection of the engines can 
be made easily. For variable loads it requires great astute- 
ness and experience, nor is it safe to arg.ue from experience 
based on other kinds of variable service. No engines can be 
subject to greater variations of load than are met in marine 
engines driving a ship in a high sea. If the screw rises from 
the water the whole load is thrown off, and resumed again with 
terrible violence when the screw is submerged. Nevertheless 
an engine, which is so arranged as to perform well under these 
trying circumstances, might perform badly when put on 
electric railway or power service, not because of its inability 
to stand the far less severe changes of load, but for the reason 
that the average load would be much further from its full 
capacity than in the case of marine practice. For large rail- 
way and power service it is best to use direct connected units, 
for the sake of compactness and economy. If a station is of 
sufficient magnitude to employ four or five 500 HP engines, 
direct connecting is advisable in nearly every case. 

It has been said that such a plant has a lack of flexibility 
that is dangerous in case of sudden and great variations of 



326 ELECTRIC TRANSMISSION OF POWER. 

load. This is not true if the engines have been intelligently 
proportioned for the work they have to do, although in some 
cases there has been trouble due to the fact that the engines 
were ill-fitted to operate successfully under the changes of 
load to which they were subjected. As a matter of economy 
both in engines and dynamos, it is desirable to work direct 
coupled plants at a fairly high speed. There is no need of ex- 
aggerating the size of both engine and dynamo for the sake of 
running at 50 to 70 revolutions per minute, when equally good 
engines and dynamos of smaller size and less weight can be 
obtained by running at 90 to 120 revolutions or more. Much 
of the unwieldiness charged against large direct coupled units 
has been the result of yielding to the importunities of some 
engine builder who wanted to sell a very large machine, and 
putting in an engine and dynamo working at absurdly and un- 
necessarily low speed. 

Electric power transmission, with a steam engine as the 
prime mover, is most likely to be developed in the direction of 
very large plants, to which these remarks apply most forcibly, 
particularly as in order to make transmission of power from 
a steam-operated station profitable, it is necessary to seek the 
very highest efficiency. Apart from the cost and inconvenience 
of very low speed units, it must be borne in mind that the 
mechanical efficiency of large low speed engines with heavy 
pistons and enormous fly-wheels, is lower than that of those 
designed for more reasonable speeds, which gives added reason 
for moderation in planning direct coupled units. 

Throughout the design of a power station the probabiHty of 
light loads must be considered. Not only does this have an 
important bearing on the economy of the engines, but it influ- 
ences that of the boilers as well. The cost of operation de- 
pends on the coal consumption, and this in turn not only on 
the amount of steam that must be produced, but on the effi- 
ciency of its production. 

There is, however, no classification of boilers on which one 
can safely rest in judging of their economy. There is much 
more difference in economy between a carefully fired and a 
badly fired boiler of the same kind, than there is between the 
best and the worst type of boiler in ordinary use. Boilers may 



ENGINES AND BOILERS. 327 

be generally divided into three classes : Shell boilers, in which 
the water is contained in a plain cylindrical tank heated 
on the outside ; tubular boilers, in which there are one or 
many tubes running lengthwise of the boiler shell, and serving 
as channels for the heated gases from the fire ; and water-tube 
boilers, in which the water is contained in a group of metallic 
tubes, around which the heat of the fire freely plays. In some 
tubular boilers the structure is vertical, with a furnace at the 
bottom, and the tubes are numerous and rather small, giving a 
large heating surface. Tubular boilers are also very often 
arranged horizontally, and in one very excellent and common 
type (return tubular), the flame and heated gasses pass horizon, 
tally under the boiler shell and then back through the tubes to 
the furnace end and thence upward into the stack. A typical 
water-tube boiler is shown in Fig. 190. Here the furnace is at 
the left of the cut and the stack at the right. The tubes are 
inclined as is usual in water-tube boilers, and steam space is 
secured by the drum above. Each class of boiler has nearly as 
many modifications as there are makers, most of them being 
with relation to the arrangement of the fire with respect to 
the boiler proper. Boilers are not altogether easy to compare 
on account of differences in rating. The usual HP rating is 
very vague, and does not in the least correspond with the per- 
formance, being based on steam pressure lower than is now 
normal. Specifications should demand actual evaporative capac- 
ity under the exact conditions of the proposed use, including, 
if the boilers are to be at times forced, the conditions then to be 
fulfilled. 

As to the merits of the different classes, opinions differ very 
widely. It is clear from experience that the simple shell 
boiler is decidedly inferior to either of the others in econ- 
omy, in spite of its simplicity and cheapness. Of late years it 
has been the fashion to employ water-tube boilers under all 
sorts of conditions, on account of their supposed great effi- 
ciency as steam producers, safety, and compactness. Purely 
experimental runs with such boilers often show phenomenal 
efficiency, but tests under working conditions sometimes re- 



328 



ELECTRIC TRANSMISSiON OP POWER. 



suit otherwise. It is important to note that not only does 
skill in firing produce a great improvement in boiler economy, 
but that by influencing the firing different kinds of coal give 
^ very different results quite independent of their theoretical 
value as fuel. The thermal value of coal, or other solid fuel, 
is almost directly as the proportion of carbon contained in it, 
and for comparative purposes boiler tests are generally re- 
duced to evaporation of water from and at 212° F. per pound 
of combustible used, i.e., per pound of carbon. However, the 
firing in different furnaces is differently affected by changes in 




Y/W///////////////////////, 

Fig. 190. 



fuel, SO that it is impossible to predict by tests on one boiler 
what a similar one will do under other conditions. 

Altogether, the subject of boiler efficiency is a difficult and 
tangled one, since the conditions are constantly changing, 
and the best guide is found in the general result of a long series 
of tests rather than in theories of combustion. Forcing the 
output of a boiler usually injures its efficiency by compelling 
the combustion of an abnormal amount of coal for the grate 
surface of the furnace. It follows that a boiler is apt to be 
more efficient at moderate loads than at very high ones. In 
marine practice, boilers may sometimes have to be forced to 
a high output to save weight and space. In electric stations 



ENGINES AND BOILERS. 



329 



it is sometimes better to force the boilers at the hours of heavy- 
load, than to keep a relay of boilers banked in readiness for 
use, but except for this, the boilers, like the rest of the plant, 
should be worked as near their maximum efficiency as possible. 
The best fuel to use is not at all invariably that of the 
highest thermal value, in fact v/ith the proper furnace a grade 
of coal only moderately good is very often the most economic 
cal. In starting a steam plant of any kind comparative tests 
of various coals should generally be made, and are more than 
likely to pay for themselves many times over. In absolute 
heating value various kinds of fuel compare about as follows: 



Kind of Fuel. 



Best anthracite . . 
Welsh steam coal 

Pocahontas 

Cumberland 

Coke, ordinary . . 

Cape Breton 

Lignite 

Peat, dry 

Wood, dry 



Heat of Combustion. 


Evaporati©n. 


15,250 


15.8 


14,500 


16.0 


14,375 


14.9 


13,750 


14.2 


12,760 


13.2 


12,500 


13.0 


11,750 


12.2 


9,650 


10.0 


7,250 


7.5 



The heat of combustion is per pound of fuel, and is given in 
thermal units, this unit being the heat required to raise 1 lb. 
of water 1° F. 

The evaporation gives the pounds of water which can be 
evaporated from and at 212° F. by the complete utilization 
of the annexed heats of combustion. In other words, no more 
than 15 lbs. of water can possibly be evaporated by 1 lb. 
of coal of the thermal value of 14,500. Extravagant claims 
are sometimes made for patented boilers of strange and 
imusual kinds, so it is well to bear these figures in mind and 
to remember that you cannot evaporate more water than 
the figures indicate, any more than you can draw a gallon 
out of a quart bottle. In practice coal is likely to fall perhaps 
10 per cent below the thermal values given above. Good 
boilers with careful firing will utilize from 70 to 75 per cent 
of the thermal value of the coal. Occasional experimental 
runs may give slightly higher figures, but only under very 
exceptional circumstances. 



330 



ELECTRIC TRANSMISSION OF POWER. 



Now as to actual tests under boilers. Examinations of 
more than a hundred carefully conducted tests by various 
authorities show from 8 to 13 lbs. of water evaporated 
from and at 212° per pound of combustible. As average good 
steam coal contains from 8 to 15 per cent of ash and mois- 
ture, these results correspond to from 7 to llf lbs. of water 
per pound of coal. Now and then a single test gives a 
result a few hundredths of a pound better than 13 lbs. per 
pound of combustible, and an occasional poor boiler shows less 
than 8 lbs. Generally from 10 to 15 lbs. of coal are consumed 
per hour per square foot of grate surface. The following 
table gives a general idea of the results of boiler tests, good, 
bad, and indifferent. 



Kind of Boiler, 


Kind of Coal. 


Evaporation. 


Return tubular 


Welsh steam . . 


13.12 


Water-tube 


( Bituminous, 
1 pea and dust 
Cumberland. 


3 parts / 

, 1 part j' • • • • 


13.01 


Return tubular 


12 47 


Vertical tubular 


Cumberland 


12.29 


Return tubular 


Cumberland 


12.07 


Return tubular ... 


Cumberland 


12 03 


Return tubular 


Anthracite 


11.63 


Marine . 


Newcastle .... 


11.44 


Water-tube 


Anthracite 


11.31 


Water-tube 


Cumberland 


10.98 


Plain tubular 


Anthracite 


10 88 


Water-tube 


Cumberland 


10.79 


Marine 


Welsh steam 


10.44 


Return tubular 


Anthracite 


10.43 


Locomotive 


Coke.... 


10.39 


Water-tube 


Anthracite 


10 00 


Return tubular 


Anthracite 


9.55 


Cylinder 


Anthracite 


9.22 


Cylinder 


Cumberland 


8.74 




Anthracite 


8.44 









The evaporation is per pound of combustible. The most 
striking feature of this table is the small difference in efficiency 
between the various kinds of boiler. Putting aside the cylin- 
drical shell boilers, which are distinctly inferior to the others, 
it appears that in other types of boiler there is little to choose 
on the score of economy alone. The difference between the 
better and worse boilers of each class, due to difference of 
design, condition, and firing, is much greater than the differ- 



ENGINES AND BOILERS. 331 

ence between any two classes. Even the same boiler with 
different fuels, firing, or when in different condition, may give 
evaporative results varying by 30 per cent. Economy de- 
pends vastly more on careful firing and proper proportion- 
ing of the grate and heating surfaces to the fuel used, than 
upon the kind of boiler. In fact, judging from all the available 
tests, the differences between various types of boiler when 
properly proportioned are quite small. 

The most that can be said is that plain shell boilers are de- 
cidedly inferior to the other forms, of which the horizontal return 
tubular and the water-tube have given slightly higher results 
than the others. Water-tube boilers are generally rather com- 
pacter and stand forcing better than ordinary tubular boilers. 
They also are less likely to produce disastrous results if they 
explode. On the other hand, they are more expensive, and are 
as a class hard to keep in good condition, particularly if the 
water supply is not of good quality. 

Probably under average conditions a well-designed horizon- 
tal return tubular boiler will give as great evaporative effi- 
ciency as can regularly be attained in service, and the choice 
between it and a water-tube boiler is chiefly in economy of 
space and capacity for forcing. There is no excuse for the 
explosion of any properly cared for boiler. 

The actual evaporation secured per pound of total fuel is 
something quite different from the figures in the table just 
given. In the first place, allowance must be made for ash and 
fuel used for banking the fires. In the second place, in regular 
rimning the firing is seldom as careful as in tests. 

On account of these the evaporations per pound of com- 
bustible given in the table must be reduced from 15 to 20 per 
cent to correct the result to pounds of coal used in actual 
service. 

Ten pounds of water or over, evaporated from and at 212° per 
pound of total fuel may be regarded as exceptionally good prac- 
tice in every-day work. Nine to 10 lbs. under the same con- 
ditions represents fine average results, and 8 to 9 lbs. is much 
more common. In fact, 8 lbs. is an unpleasantly frequent figure, 
particularly in boilers operated under variable load, such as is 
generally found in electric plants of moderate size. All these 



332 



ELECTRIC TRANSMISSION OF POWER. 



facts point out the necessit}^ of thorough and careful work in 
every part of a power plant. Bad design or careless opera- 
tion anywhere plays havoc with economy. In most instances 
far too little attention is paid to the adaptation of the furnace to 
the particular fuel used. In case of attempting power trans- 
mission from cheap coal at or near the mines, the furnace and 
firing problem is of fundamental importance. Most furnaces 
are constructed to meet the requirements of high grade fuel 
and are quite likely to work badly with anything else. In 
transmitting power from cheap coal the grate surface, draft, 
and so forth must be carefully arranged with reference to the 
grade of fuel to be used and not with reference to standard 
coals used elsewhere. The methods of firing, too, require 
careful attention. 

At the present time mechanical stokers are in very extensive 
use in some parts of the country. The reports from them are 
of varying nature, but the consensus of opinion seems to be 
that they are very advantageous in working medium and low 
grade coals, but of less utility in the case of high grade 
coals. They are somewhat expensive and require intelligent 
care now and then like all other machinery, but when it comes 
to firing large amounts of cheap fuel at a fairly regular rate 
they do m.ost excellent work. When coal is dear, careful hand 
firing is probably more economical than any mechanical 
method. With first-class coal and boilers one good fireman 
and a coal-passer can take care of 2,000 KW in modern appara- 
tus, so that the total cost of firing is not a very serious matter. 
A poor fireman is dear at any price, and quite as disadvantage- 
ous to the station as a poor engineer. 



Kind of Engine. 


Coal per IHP Hour. 


Condensing. 


Non-Condensing. 


Simple, dependent valve 


2.77 
2.33 
2.22 
2.00 
1.88 
1.66 
1.44 


3.66 


Simple, independent valve 


3.11 


Compound, dependent valve 


2.66 


Compound, independent valve 


2.44 


Triple, dependent valve 




Triple independent valve 




Triple, independent valve large 









ENGINES AND BOILERS. 333 

Reverting now to engine performances, we may form a fairly 
definite idea of what may ordinarily be expected in the way of 
coal consumption per indicated horse-power hour. 

The foregoing table shows the coal consumption of the 
various kinds of engines, based on the burning of 1 lb. of 
coal for each 9 lbs. of feed water used. Although greater 
evaporation can often be obtained, 9 lbs. of water per pound 
of coal is a very good performance indeed, decidedly better 
than is found in general experience. It presupposes good 
boilers, good coal, and skilful firing, such as one has a right 
to expect in a large power plant. 

The figures apply only to engines of several hundred HP, 
at or near their points of maximum economy, and operated 
from a first-class boiler plant. 

They can be and are reached in regular working, and are 
sometimes exceeded. A com.bination of great efficiency at 
the boilers and small steam consumption in the engine some- 
times gives remarkable results. The best triple expansion 
condensing engines worked under favorable conditions can be 
counted on to do a little better than 1.5 lbs. of coal per IHP 
hour, occasionally even in the neighborhood of 1.25 lbs. Even 
with compound condensing engines, tests are now and then 
recorded, showing below 1.5 lbs. of coal per IHP hour. But 
these very low figures are the result of the concurrence of 
divers very favorable conditions, and those just tabulated 
are as good as one should ordinarily expect. It must not be 
supposed that the weight of coal used per HP hour necessarily 
determines the economy of the plant. The cost of fuel of 
course varies greatly, and its price in the market is by no 
means proportional to its thermal value. As a rule, the coals 
which give the best economic results are not those of the 
greatest intrinsic heating power. On the contrary, dollar for 
dollar, the best results are very frequently obtained from cheap 
coal, or mixtures of inferior coal with a portion of a better 
grade. Hence, the boilers of a plant which is a model of econ- 
omy may show an evaporation of only 7 or 8 lbs. of water 
per pound of coal. Boiler tests with the conditions of economy 
in view are of great importance, and are likely to pay for them- 
selves tenfold in even a few months of operation. 



334 ELECTRIC TRANSMISSION OF POWER. 

A word here about fuel oil. Petroleum has, weight for 
weight, much greater heating power than coal. Its heat of 
combustion is about 20,000 to 21,000 thermal units, it costs 
little to handle and fire, leaves no ash and refuse to be taken 
care of, produces little smoke, and is generally cleanly and 
convenient. 

It has been thoroughly tried by some of the largest elec- 
trical companies in this country, and at moderate prices, a 
dollar a barrel or less, is capable of competing on fairly even 
terms with coal. But experience has shown some curious 
facts about its performance. The amount of steam or equiva- 
lent power required to inject and vaporize the oil in one of 
the most skilfully handled plants in existence amoimts to no 
less than 7 J per cent of the total steam produced. And 
curiously enough, the cost of oil for firing up a fresh boiler, 
and the time consumed, compare unfavorably with the results 
obtained from coal. In spite of the great amount of heat 
evolved from fuel oil, it appears to be less effective than coal 
in giving up this heat to the boiler by radiation and convection. 
There is good reason to believe that more than half the total 
heat of combustion of incandescent fuel is given off as radiant 
heat, and most of the remainder is of course transferred by 
convection both of heated particles of carbon and of molecules 
of gas. 

It is not unlikely, therefore, that a petroleum fire with its 
small radiating power and comparative absence of incan- 
descent particles, fails in economy through inability to give 
up its heat readily. This view of the case is borne out by the 
facts above cited and by the abnormally high temperature of 
the escaping gases often found in boiler tests with petroleum 
fuel. At all events it is clear from such tests that the evapo- 
ration obtained from fuel oil is not so great as would be ex- 
pected from its immense heat of combustion, and unless at 
an exceptionally low price, its use is less economical than that 
of coal. 

The most striking innovation of recent years in the genera- 
tion of mechanical power by steam is the development of the 
steam turbine. Year by year during the past decade it has 
slowly grown from experiment to realization, until at the pres- 



ENGINES AND BOILERS. 335 

ent time it has reached a position that demands for it most 
serious consideration. It looks very much as if, for many pur- 
poses, the reciprocating steam engine might be hard pushed. 

Strangely enough the steam turbine or impulse wheel is the 
earliest recorded form of steam engine, dating clear back to 
Hero of Alexandria, who flourished about 130 B.C. The engine 
which Hero suggested was merely a philosophical toy, and it 
took nineteen centuries beyond his day to produce any engine 
that was not a toy, but now after two thousand years Hero's 
idea has borne fruit. 




Fig. 191. 

The fundamental principle of the steam turbine is just that 
of the water turbine — directing fluid pressure against a series 
of rotating buckets. The first practical steam turbine, devised 
by De Laval, is very closely akin to the Pelton water-wheel 
and to the little water-motors sometimes attached to faucets 
for furnishing a small amount of power. The essential fea- 
tures of his apparatus are well shown in Fig. 191. It consists 
of a narrow wheel A with buckets aroimd its periphery, re- 
volving within a housing B and supported by a rather long and 
slender shaft. Bearing upon the buckets at an acute lateral 
angle is the steam jet E, in this case one of three equidistant 
jets playing on the same wheel. To obtain the most efficient 



336 ELECTRIC TRANSMISSION OF POWER. 

working of the jet the steam nozzle is somewhat contracted at 
D, a Httle way back from the buckets. The steam is discharged 
on the other side of the wheel as shown. It strikes the buckets 
as a jet at great velocity, and should, if the conditions were 
just right, expend nearly all its energy in driving the v/heel and 
should itself leave it at or near zero velocity. Of course this 
condition does not hold in practice, but still a steam turbine of 
this De Laval construction is capable of doing marvellously well. 
The main objection to this form is the enormously high rotative 
speeds necessary for efficient running. Here, as in hydraulic 
impulse wheels, the peripheral velocity should be about one-half 
the spouting velocity of the fluid. With high pressure steam 
this is, when one works the turbine condensing, 3,000 to 5,000 
feet per second. In practice these De Laval wheels have usu- 
ally been geared to a driving shaft, but the wheel itself has run 
at 10,000 to 30,000 r.p.m., seldom below the former figure even 
in large sizes. But the economy reached has sometimes been 
very high, as in some tests a few years ago in France, when, 
with an initial pressure of 192 lbs., a 300 HP turbine showed 
a steam consumption of only 13.92 lbs. per effective HPH — 
a figure seldom reached with engines. The governing is by 
throttling the steam supply in response to the movement of 
a fly-ball governor of the kind generally familiar in steam 
engineering. 

The inconvenience of the very high rotative speed of such 
turbines has led to the development of forms working more 
along the lines of hydraulic turbines, of which by far the best 
known is the Parsons turbine, which has recently made so 
striking a record in marine work, having been applied to sev- 
eral British torpedo-boats and even to larger vessels. In this 
remarkable machine the passage of the steam is parallel to 
the axis of rotation instead of tangential, and its hydraulic 
prototype is the parallel-flow pressure turbine, shown in dia- 
gram in Fig. 200. In fact the steam is passed successively 
through a large number of such parallel-flow turbines, gradually 
expanding and giving up its energy to the successive runners 
located, of course, on the same shaft. The course of the ex- 
panding steam is well shown in Fig. 192, which gives in diagram 
its progression through four sets of vanes, two, 1 and 3, being 



ENGINES AND BOILERS. 337 

rings of guide blades, and the others, 2 and 4, rings of runner 
blades. The steam, starting at pressure P, expands succes- 
sively to Pi, Pii, Piii, Pjv, expanding sharply against the runner 
blades and giving them a reactive kick as it leaves. In this 
case the steam velocity against, say the runner blades 2, is 
not, as in the De Laval form, the full spouting velocity due 
to the initial head of steam, but merely that corresponding to 
the differential pressure P - P^. This enables the peripheral 
speed of the runner to be kept within reasonable limits without 
violating the conditions of economy, but the turbine at best 
is not a slow-speed machine. 

Fig. 193 is a longitudinal section through the Parsons 
type of steam turbine as developed in this country by the 



ccccccccc 



Stationary\Blade8 

Moving Blades 



rK 



CCCCCCCCC 

)) )) )) )) ^3) )) Jl 



Stationary\Blades> 

Moving Blades 



P4 ' 

Fig. 192. 

Westinghouse Machine Co. The steam enters from the supply 
pipe controlled by the governor and comes first into the annu- 
lar chamber A at the extreme left-hand end of the runner. 
The runner blades are graduated in size so that the expanding 
steam may give nearly a uniform useful pressure per blade, 
and to this end the diameter of the runner hub is twice in- 
creased as the steam expands towards the exhaust chamber B. 
The endwise thrust of the steam entering the turbine from A 
is balanced by its equal pressure on the balancing piston C, 
which revolves with the runner. To the left of this is an 
annular steam space and a second balance piston C. Now, 
when the expanding steam has passed the first section of the 
runner into the steam space E, it can flow back through the 
channel F and the post D against this second balance piston. 



338 



ELECTRIC TRANSMISSION OF POWER. 




Still further to the left is a second steam space and a third 
piston C, which is similarly exposed to the pressure from G, 
where the third stage of expansion begins. The effect of this 



ENGINES AND BOILERS. 339 

balancing system is to render the end thrust neghgible what- 
ever may be the ratio of expansion in the turbine. A thrust 
bearing at H keeps the working parts positioned and takes up 
the trivial thrusts which may incidentally be present. J, J, J 
are the bearings, which are out of the ordinary in that within 
the gun-metal sleeve that forms the bearing proper are three 
concentric sleeves fitting loosely. The clearance between 
them fills with oil, cushioning the bearings. Now if the run- 
ner is not absolutely in balance there is a certain flexibility in 
the bearings so that the runner can rotate about its centre 
of gravity instead of its geometrical centre, thus stopping 
vibration. An equivalent expedient is found in the De Laval 
turbine, the shaft of which is deliberately made slightly flexible 
so that it may take up rotation about its centre of gravity. 
K is Si pipe which again takes up the work of keeping the 
pressure balanced by connecting the .exhaust chamber B with 
the steam space behind the last balance piston. At M is a 
simple oil pump taking oil from the drip tank N and lifting it 
to the tank 0, whence it is distributed to the bearings. A by 
pass valve P turns high pressure steam directly into the steam 
space E, in case a very heavy load must be carried, or a con- 
densing turbine temporarily run non-condensing. R is a 
flexible coupling for the driving shaft, and at that point too is 
the worm gear that drives the governor. The governor in 
its operation is somewhat peculiar. Instead of throttling the 
steam supply so as to reduce the effective pressure, the steam 
is always sent to the turbine at full boiler pressure, but discon- 
tinuously. The main steam valve is controlled by a little 
steam relay valve which is given a regular oscillatory motion 
by a lever driven from an eccentric. The steam is thus ad- 
mitted to the turbine in a series of periodic puffs. Now the 
fulcrum of this valve lever is movable and is positioned by 
the fly-ball governor, so that the position of the valve with rela- 
tion to the port is varied without changing the rate or ampli- 
tude of the valve motion. Hence the length of the puffs is 
changed so that while at full load steam is on during most of 
the period, at light load it is on for only a small part of each 
period. This is well shown graphically by Fig. 194, which is 
self-explanatory. 



340 



ELECTRIC TRANSMISSION OF POWER. 



The governor balls are so arranged that they work both 
ways, their mid-position corresponding to full admission of 
steam, so that a violent overload can be made to shut off steam, 
and a break in the governor driving gear will do the same 
instead of letting the turbine run away. 

These turbines are capable of operating with really remark- 
able efficiency. Fig. 195, from the makers' tests, gives the 
performance, both condensing and non-condensing, of a West- 
inghouse-Parsons turbine directly coupled to a 300 KW 
quarter-phase alternator giving 440 volts at 60^, the speed be- 
ing 3,600 r.p.m. Operated condensing, the steam consumption 




WHEN RUNNING LIGHT LOAD 



Fig. 194. 



at full load falls to about 16.4 lbs. per electrical HPH, and is 
below 20 lbs. from 125 HP up. This extraordinary uniformity 
of performance at large and small loads is mainly due to the 
very small frictional losses in the turbine, although it is helped, 
perhaps, by the load curve of the generator. The results when 
working non-condensing are very much inferior to these, rela- 
tively worse than in an ordinary steam engine. 

Altogether it is an admirable showing for the steam turbine. 
The writer believes Fig. 195 to be entirely trustworthy, as it 
corresponds very closely with certain independent tests now 
in his possession, from another turbine of the same capacity 
and speed, in which tests the makers of the turbine had no 




PLxVTE XI. 



ENGINES AND BOILERS. 



341 



part. The substance of the matter is that the steam turbine 
will work just about as efficiently as a first-class compound 
condensing engine, and can not only be more cheaply made, 
but takes up much less room. In the same way, for electri- 
cal purposes a directly connected generating set with steam 
turbine is, or ought to be, much cheaper and smaller than 
those now in use, while retaining equally high efficiency. 
High rotative speed is by far the cheapest way of getting out- 



r-15000-r-150 




50 75 100 125 150 175 200 225 250 275 
electrical h0f4se power 
Fig. 195. 



325 350 375 400 425 



put, and when, as in this case, no heavy reciprocating parts 
are involved, there is no good reason for objecting to high 
speed. The present fashion for low speed dynamos is largely 
a fad, having its origin in direct coupling to Corliss engines, 
and with the modern stationary armature construction there is 
no reason why high rotative speed should not be used, at least 
in alternators. 

In Plate XI is shown the first large turbine-driven generator 
installed for regular commercial service in this country. Smaller 
ones had been in use in isolated plants for some time, but 



342 ELECTRIC TRANSMISSION OF POWER. 

this 1,500 KW set, installed for the Hartford Electric Light 
Co., was the first important installation of this kind. The 
turbine is designed for a maximum output of 3,000 HP at a 
speed of 1,200 r.p.m., and the complete set, weighing only 
175,000 lbs., takes a floor space of but 33' 3'' X 8' 9". The 
generator is a quarter-phase machine at 60^ frequency. This 
outfit should be capable of giving an efficiency rather better 
than that shown in Fig. 195 — probably less than 15 lbs. of 
steam per electrical HPH at steady full load ; in other words, it 
should do nearly as well as a triple expansion engine. This 
machine has now been in successful operation for some four 
years. 

Alternators may be conveniently and cheaply built for the 
speed impHed in steam turbine practice, but continuous cur- 
rent generators involve some difficulties. For power trans- 
mission work turbo-generators have much to recommend them 
as auxiliaries, and there is a strong probability of their taking an 
important place in the development of the art- There has not 
yet been accumulated enough experience with them to enable 
a final judgment of their practical properties to be formed, 
or to justify an unqualified indorsement of their economy. 

Another successful form of steam turbine, now consider- 
ably used in units of output as great as several thousand KW, 
is the Curtis, which differs radically in several respects from 
that just described. In the first place it is not a pressure 
turbine but an imxpulse turbine, in which the steam is expanded 
in the admission nozzles to a high jet velocity, the kinetic 
energy of which is then utihzed in the runner buckets. It 
is thus dynamically more akin to the De Laval than to the 
Parsons turbine, but differs from it much as a true impulse 
turbine differs from a Pelton wheel. 

In the second place it is regularly built in all the larger 
sizes as a vertical shaft machine carrying the generator arma- 
ture on its upper end and being supported below by an inge- 
niously contrived water step kept afloat by a pressure pump. 
In the Curtis turbine, however, the expansion takes place 
not in a single nozzle as in the De Laval type but in several 
successive stages so that the jet velocity and with it the neces- 
sary peripheral speed is very considerably reduced. Fig. 196 



ENGINES AND BOILERS. 



343 



shows the course of the steam and is almost self-explanatory. 
The upper section is the first stage, the lower the second stage, 
and in some of the larger units three or four stages are used. 
Their effect is akin to that of compounding an engine in the 
better temperature distribution and proportioning of parts. 
The governing in this turbine is exactly in line with the prin- 



ts £:<sor>-> i:T/-)est. 







maimiaaa<mm 



A/ozzftS 

St.ot./'on<ary-/ J3/t£n=/&^ 
A/7o >^/m^ J3/crc:/e s 
SLat/'or-tar-c/ J3/<:ye=/&^ 




S'/n 



Movn^ 3/aie/ss 






cccccccccccccccccccccc 



A^oi/'/Dg /3/ctc/GS 






|(< «(<«(« «(««(««<« 



AffO V/TT^ /D/tSC^G 3 



1 1 1 I I 1 

Fig. 196. 

ciples of hydraulic impulse turbines; the numerous admission 
nozzles being fitted with independent valves as shown in the 
cut. These are in succession opened or closed in accordance 
with the requirements of the load, by the action of a sensitive 
fly-ball governor which controls a series of relay valves, in turn 
working the admission valves. In the earlier and some of 



344 ELECTRIC TRANSMISSION OF POWER. 

the later turbines these relay valves have been electrically 
actuated, but at present both this and purely mechanical con- 
trol are used. As each admission valve is either fully open or 
closed there is no throttling of the steam, which gives a material 
gain in efficiency. 

Plate XII shows a 500 KW Curtis turbo-generator which 
is peculiarly interesting as being a direct current machine 
designed for railway purposes. The rotative speed is 1,800 
r.p.m., but in spite of this, the problem of commutation has 
been successfully met. In this, as in all the large Curtis turbines, 
there is but one supporting bearing on which the moving parts 
spin top fashion kept in line by a pair of small guide bearings. 
The structure is thus wonderfully compact, but it is still an 
open question among engineers as to the advisability of a 
vertical shaft. It gains a little in friction and a certain amount 
in space together with immunity from flexure of shaft, while 
on the other hand it loses greatly in accessibility, and exposes 
the generator portion to certain risks from heat, steam, and oil 
that are not altogether negligible. From a practical stand- 
point the Curtis turbine has made a good record, and many 
large units, even up to 5,000 KW, are in successful use, to no 
small extent in large stations designed for railway service, 
polyphase current being generated and transmitted to con- 
verter stations. The large polyphase turbo-generators are all 
of the vertical type closely resembling Plate XII. 

The strong points of steam turbines are cheapness for a 
given output, economy of floor space, freedom from vibration, 
uniform efficiency at various loads, and light friction. Of 
these the first is the direct result of high speed both in turbine 
and generator. For some years there was a strong tendency 
toward very low engine speeds, which produced generating 
units of needlessly great weight and cost. The turbine goes 
to the other extreme of speed, and while it runs at speed too 
high for the most economical construction, can be built in- 
cluding the generator at a cost probably below that of any 
other direct connected unit. The current price is relatively 
high, being adjusted to that determined by engines, but this 
condition is of course temporary. Economy of floor space is 
very marked, especially in the vertical shaft type, but it 




PLATE XII. 



ENGINES AND BOILERS. 845 

actually is much less than at first appears, since the location 
of the boilers generally determines the area of the plant, and 
turbines cannot conveniently be huddled into the space which 
their dimensions would suggest. They are remarkably free 
from vibration, due to the necessity of avoiding centrifugal 
strains by extremely careful balancing, and their friction is 
very light indeed. The uniform economy at various loads is 
partly due to small friction and partly to the fact that the 
expansion of the steam is substantially fixed by the construc- 
tion and does not vary materially with the load. Nevertheless, 
as has already been shown, an engine properly designed for 
varying load will show at least equally good practical results. 
Compare in this the lowest curve of Fig. 186, and Fig. 195, to 
say nothing of Fig. 187. 

The actual efficiency of the steam turbine is good without 
being in any way phenomenal. At equal steam pressure, super- 
heat, and vacuum, the steam turbine, in its present stage of 
development, is as a rule slightly less efficient per brake horse- 
power than a first-class compound condensing engine. Tur- 
bines, however, suffer extremely, like other engines in which 
there is very great expansion, from diminished vacuum and 
only by using a vacuum of 2S" and 100° to 150° F. superheat- 
ing, can they be brought up to a performance equivalent to a 
compound condensing engine. 

Very recently turbines carrying the expansion to as many as 
five stages have been introduced, and from large units of this 
construction it has been possible to secure economy comparable 
with that of first-class triple expansion engines and far surpass- 
ing that shown by the earlier turbines here described. 

As a practical matter, however, one can very comfortably 
stand some loss in efficiency if thereby the fixed charges and 
maintenance can be kept down. The greater the necessary 
proportion of such items, the more complacently can one 
stand a slight loss in steam economy. In the case of engines 
which from the conditions of their use give a rather small 
annual output for their capacity, reduction of fixed charges 
and maintenance is of great importance. Hence for auxiliary 
plants in power transmission which are idle a large part of the 
time, there is a great deal to be said for the turbo-generator if 



846 



ELECTRIC TRANSMISSION OF POWER. 



it can be had at a reasonable figure. It can also be put into 
action more quickly than ordinary engines, and handles heavy 
overloads well beside running easily in parallel by reason of 
the uniform rotative effort. 

Considerable space has here been given to describing some 
of the details of steam turbines, in the belief that they are of 



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1- 

O 

1500 






















i 

looe 

500 




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bp..Mi 


a 


NOON 


6 





Fig. 197. 



sufficient importance to warrant it even in a chapter not 
intended to be in the least a compendium of steam practice, 
but a mere outHne of the essential facts. They certainly have 
already proved their right to a place, and the question is now 
merely that of the probable limitations of their usefulness, 
which only protracted experience can disclose. 

For an electrical power station operated by steam power the 
vital economical question is the cost of fuel per kilowatt hour, 



ENGINES AND BOILERS. 



347 



rather than the performance of engines and boilers alone. 
This final result involves the performance of the station appa- 
ratus under varying loads, too frequently rather light, and, 
impHcitly, the skill of the operator in keeping his apparatus 
actually running as near its point of maximum economy as 
possible, in spite of changes in the electrical output. This 
personal element forbids a reduction of the facts to general 
laws, but a concrete example will be of service in showing what 
may be expected in a well-designed and well-operated power 
plant. Fig. 197 shows a pair of '' load lines," from a large and 
particularly well-operated power plant. The solid line shows 



±u 




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3 




\ 


















O 

I 






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^ 






V 


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-J 
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the variations of load throughout a day in the latter end of 
January, and the broken line the variations of load during a 
day early in April. 

The early darkness of a winter's day is very obvious in the 
former line. The station carries in addition to lights a heavy 
motor service that keeps up a fairly uniform load through- 
out the day, until the sudden call for lights in the early 
evening. The load factor shown by the solid hne is .35 (i.e., 
this is the ratio between maximum and average load). The 
second load line gives a much better relation between these 
quantities, the load factor being .64, which is quite usual in 
this station during the spring and summer. Of course every 
effort is exerted to keep the machines which are in use as fully 



348 ELECTRIC TRANSMISSION OF POWER. 

loaded as possible. In spite of this the small output during 
the early morning hours, coupled with the losses due to circu- 
lating pumps and other minor machinery, and the fuel used for 
banking and starting fires, brings the cost of fuel during this 
period far above the average for the day. The curve in Fig. 
198 shows roughly the variation in the cost of fuel per KW- 
hour throughout the day, taken from the average of a num- 
ber of tests. As the fuel cost in a large central station is a 
considerable portion of the total expense, it is evident that the 
result is an excellent one. During all the hours of heavy load 
the cost of fuel is less than six-tenths of a cent per KW-hour, 
and the total cost of production but little more. This result 
will give an excellent idea of the cost of generating power on 
a large scale with cheap coal. It is, however, exceptionally 
good, and can only be equalled by a very well managed plant 
with the best modern equipment both electrical and mechanical. 
Of course the expenses of distribution, administration, and 
the like must be taken into account in considering the cost 
per KW hour delivered. The general question of station ex- 
penses cannot be here investigated, but this brief digression 
gives some idea of the necessary relation between the character 
of the work and the commercial results in generating electric 
power on a large scale, so far as the use of steam engines as 
prime movers is concerned. 



CHAPTER IX. 

AVATER-V/HEELS. 

The importance of the development of water-powers for 
electrical purposes we have already come fully to realize. The 
lessons of the last few years have been exceedingly valuable 
ones, and it is safe to say that the utilization of water-powers 
for electrical transmission will be kept up until every one 
which is capable of commercially successful development is 
worked to its utmost capacity. In spite of the length of time 
that water-wheels of various sorts have been used, it is only 
very recently that these prime movers have been brought to a 
stage of development that renders them satisfactory for elec- 
trical purposes. The old water-wheel was even more trouble- 
some as a source of electrical power than the old slide valve 
steam engine. 

The customary classification of water-wheels for many years 
has been into overshot, undershot, and breast- wheels, and 
finally turbines. Various modifications of all these have, of 
course, been proposed and used. Of these classes, the first 
three may be passed over completely as having no importance 
whatever in electrical matters, save in certain modifications so 
different from the original wheel as to be scarcely recognizable. 
To all intents and purposes they are never used for the pur- 
pose of driving dynamos, although occasionally an isolated 
instance appears on a very small scale. 

It is the turbine water-wheel which has made modern 
hydraulic developments possible, and more particularly elec- 
trical developments. The turbine practically dates from 1827, 
when Fourneyron installed the first examples in France, 
although it is interesting to know that a United States patent 
of 1804 shows a wheel of somewhat similar description, never 
so far as is known used. The modern turbine consists of two 
distinct parts, the system of guide blades and the runner. The 
runner is the working part of the wheel, and consists of a 

849 



350 



ELECTRIC TRANSMISSION OF POWER. 



series of curved buckets so shaped as to receive the water with 
as little shock as practicable, and to reject it only after having 
utilized substantially all of its energy. These buckets are 
arranged in almost every imaginable way around the axis of 
the runner, but always symmetrically. 

Sometimes the curvature of the buckets is such that the 
water after having passed through them leaves the wheel 
parallel to its axis; sometimes so that the water flows inward 
and is discharged at the centre of the runner; sometimes so 
that it passes outward and is discharged at the periphery. 





Fig. 200. 



More often the buckets have a double curvature so that the 
water flows along the axis and at the same time either inwardly 
or outwardly. It is not unusual, moreover, to have two sets of 
buckets on the same shaft for various purposes. The growth 
of the art of turbine building has made any classification of 
turbines depending on the direction of the flow of the water 
very uncertain, as in nearly every American turbine this flow 
takes place in more than one general direction, usually inward 
and downward. Aside from the runner the essential feature 
of the modern turbine is the set of guide blades which sur- 
roimd the runner, and which are so curved as to deliver the 
water fairly to the buckets in such direction as will enable it 
to do the most good. Accordingly these blades are curved in 
all sorts of ways, according to the way in which the water is 
intended to be utilized. 

Fig. 199, taken from Rankine, shows a species of idealized 
turbine which discloses the principles very clearly. In this fig- 
ure A is the guide blade system and B the runner. The flow 
is entirely along the axis, forming the so-called parallel flow 
turbine^ a form not in general use in America. 



WATER-WHEELS. 351 

Fig. 200 shows the sort of curvature which is given to the 
guide blades and to the buckets of the runner. The axis of 
this or any other kind of turbine may be horizontal or vertical, 
as convenience dictates. As may be judged from the illustra- 
tion, the water acts on the runner with a steady pressure, and 
the buckets of the runner are always filled with the water 
which drives them forward. Working in this way by water 
pressure due to the weight of the water column, it is not 
necessary that the turbine should be placed at the extreme 
bottom of the fall, provided an air-tight casing is continued 
below the runner so as to take advantage of the solid water 
column below the turbine. Such an arrangement is called a 
draft tube, and may be of any length up to the full column 
which may be supported by atmospheric pressure, provided 
the body of water shall be continuous so that there shall be no 
loss of head due to the drop of the water from the wheel to the 
level of the water in the tail-race. It is as if the column below 
were pulling and the column above pushing, the runner being 
in a solid stream extending from the highest to the lowest 
level of v/ater used. As a matter of practice the draft tube is 
generally made considerably shorter than the column- of water 
which might be supported by atmospheric pressure, generally 
less than 20 feet, depending somewhat on the size of the wheel. 
With longer tubes it is difficult to preserve a continuous column, 
which is necessary in order to utilize the full power of the 
water. 

Nearly all American turbines are of this so-called '' pressure" 
type. There is, however, another type of turbine wheel used 
somewhat extensively abroad, and occasionally manufactured 
in this country, which without any very great change in 
character of the structure operates on an entirely different 
principle. There are present, as before, guide blades deliver- 
ing the water to the buckets of the runner, but the spaces be- 
tween these blades are so shaped and contracted as to deliver 
the water to the runner as a powerful jet. The energy of 
water pressure is converted into the kinetic energy of the 
spouting jet, and the buckets of the runner are not filled solidly 
and smoothly with the water, but serve to absorb the kinetic 
energy of the jets, and discharge the water below at a very 



352 



ELECTRIC TRANSMISSION OF POWER. 



low velocity. Such turbines are known as impulse turbines, 
from the character of their action. In the pressure turbines the 
full water pressure acts in the runner and in the space between 
the guides and the runner. In the pressure turbine each space 
between the guide blades acts so as to form a water jet, which 
impinges fairly on the bucket of the runner without causing a 
uniform pressure either throughout the bucket spaces or in the 
space between runner and guides. It is not intended that the 
passages of the wheel should be, as in the pressure turbine, 
entirely filled with the water, nor is it best that they should 
be. Fig. 201 gives a sectional view from Unwin showing the 
arrangement of the guide blades and buckets of an impulse 




Fig. 201. 



turbine, in which the flow is, as in the pressure turbine previ- 
ously shown, in general along the axis of the wheel. An 
impulse turbine necessarily loses all the head below the wheel 
and cannot be used with a draft tube. 

Occasionally an attempt is made, in the so-called limit tur- 
bines, so to design the guides and buckets that the jets may 
completely fill the buckets, which are adapted exactly to the 
shape of the issuing stream. In such case the turbine works 
as an impulse wheel or as a pressure wheel, according as the 
draft tube is or is not used. 

A modified impulse turbine, largely used for very high heads 
of water, is found in the Pelton and similar wheels, in which 
the impulse principle is used through a single nozzle acting in 
succession on the buckets of a wheel which revolves in the 



WATER-WHEELS. 353 

same plane with the issuing jet. Such a Pelton wheel is shown 
in Fig. 202. Occasionally two or more nozzles are used, de- 
livering water to the same wheel. Impulse wheels of this class 
are exceedingly simple and efficient, and work admirably on 
high heads of water. They are, moreover, very flexible in the 
matter of obtaining efficiently various speeds of rotation from 
the same head of water, as the whole structure is so simple 
and cheap that it can be modified easily to suit varying condi- 
tions. 

It is obvious that the operation of such an impulse wheel is 
similar to that of a true impulse turbine, in which only one, 




Fig. 202. 

or at the most three or four jets from the guide blades are util- 
ized. Most of the hydraulic work done in this country is ac- 
complished with pressure turbines, which are worthy, therefore, 
of some further description. A small but important por- 
tion is accomplished by Pelton and other impulse wheels, and 
in a very few instances the impulse turbine proper has been 
used. 

There are manufactured in this country more than a score 
of varieties of pressure turbines. They differ widely in de- 
sign and general arrangement, but speaking broadly it is safe 
to say that most of them are of the mixed discharge type, in 



354 



ELECTRIC TRANSMISSION OF POWER. 



which the water passes away from the buckets of the runner 
inward and downward with reference to the axis of the 
wheel. It would be impossible to describe even a considerable 
part of them without making a long and useless catalogue. 
The essential points of difference are generally in the con- 
struction of the runner and in the mechanism of the guide 
blades. In a good many turbines regulation is accomplished 
by shifting the guide blades so as to deliver more or less water 
to the ruimer. A few types will serve to illustrate the general 
character of some of the best-known American wheels. Fig. 




Figs. 203 and 204. 

203 shows the so-called Samson turbine of James Leffel & 
Co., and Fig. 204, the runner belonging to it. This wheel is of 
the class which regulates by shifting the guide blades, which 
are balanced and connected to the governor by the rods at the 
top of the casing shown. The water enters the guide blades in- 
wardly, and the runner is provided as shown with two sets of 
buckets; the upper set discharging inwardly, the lower and 
larger set downwardly. The action of the wheel is almost 
equivalent to two wheels on the same shaft, the intention being 
to secure an unusually large power and speed from a given 



WA TER-W HEELS. 



355 



head of water on a single wheel structure. This result is, 
as might be anticipated, accomplished, and for a given diam- 
eter the Samson turbine has a speed and power consider- 
ably greater for a given head than found in the usual standard 
single wheels. As before remarked, however, it is almost, 
mechanically speaking, equivalent to two wheels through its 
pecuHar feature of double discharge through independent 
buckets. 

Another very excellent and well-known wheel is the Victor 
turbine, shown in Fig. 205. In this wheel the gate is of the so- 




FlG. 205. 



called cylinder type, which lengthens or shortens the apertures 
admitting water to the guide blades. The runner of this 
wheel is so shaped that the water is discharged inwardly 
and downwardly. The area of the runner blades exposed 
to the full water pressure is notably great. The cyhnder 
form of gate is rather a favorite with American wheel man- 
ufacturers, and is intended to secure a somewhat uniform 
efficiency of the wheel, both at full and part load, although 



356 



ELECTRIC TRANSMISSION OF POWER. 



how completely it does this is a matter which, of course, is still 
in dispute. The wheel shown, however, is an exceptionally 
good and efficient one, so far as can be judged from general 
practice. The same makers also manufacture a wheel with 
shifting guide blades. 

Another excellent wheel of the cylinder gate type, the 
McCormick, is shown in Fig. 206. The runner of this wheel has 
its main discharge downward. It has a rather large power for 
its diameter, owing to the proportion of the runners, and is 




Fig. 206. 



well known as a successful wheel considerably used in driving 
electrical machinery. 

These turbines are typical of the construction and arrange- 
ment used by first-class American manufacturers. They are 
all arranged for either horizontal or vertical axes, and for 
purposes of driving electrical machinery are whenever possible 
used in the horizontal form. All of them, particularly the two 
first mentioned, have been widely used for electrical purposes. 
They are all practically pure pressure turbines and are installed 



WATER-WHEELS. 



857 



usually with draft' tubes of appropriate length. They are 
often, too, installed in pairs, two wheels being placed on the 
same shaft, fed from a common pipe but discharging through 
separate draft tubes. The arrangement of these draft tubes 
is very various, as they can be placed in any position convenient 
for the particular work in hand. Fig. 207 shows a common 
arrangement where a single wheel is to be driven. The water 




iiiiiiiiiM 
iliiiiiiiiii 



Fig. 207. 



enters through the penstock, passing into the wheel case, 
through the wheel, which has, as is generally the case except 
with very low heads, a horizontal axis, and thence passes into 
the tail-race through the draft tube, shown in the lower part of 
the cut. The full head in the particular case shown is 43 feet, 
so that the draft tube is fairly long. Where double wheels are 
employed, there is no longer any necessity of taking up the 
longitudinal thrust of the wheel shaft, and an arrangement 



358 



ELECTRIC TRANSMISSION OF POWER. 



frequently followed is shown in Fig. 208, which gives a very 
good idea of the general arrangement of the pair of horizontal 
turbines, which may be directly coupled to the load or, as in 
the case just mentioned, drive it through the medium of belts. 
In many instances it is found cheaper and simpler to mount 
the two Avheels together in a single flume or wheel-case, so as 
to discharge into the same draft tube. Fig. 209 shows an 
arrangement which is thoroughly typical of this practice, 
applied in this case to a low head. The pair of wheels are 
here arranged so as to discharge into a common draft tube 




Fig. 208 



between them, while they receive their water from the timber 
penstock in which they are inclosed. Such wooden penstocks 
are generally very much cheaper than iron ones and for low 
heads have been extensively used. 

The central draft tube here shown need not go vertically 
downwards, but may take any direction that the arrangement 
of the tail-race requires. Whether the draft tube is single or 
double is determined mainly by convenience in arranging the 
wheel and its foundations, and the tail-race. The use of a pair 
of turbines coupled together is not only important in avoiding 



WATER-WHEELS. 



359 



end thrust, but it also enables a fair rotative speed to be 
obtained from moderate heads, which is sometimes very im- 
portant in driving electrical machinery. 

For example, suppose one desired to drive a 500 KW gener- 
ator by turbines from a 25 foot head. Allowing a little margin 
for overload the turbine capacity should be in the neighbor- 
hood of 750 HP. Now turning to a wheel table applying, for 
instance, to the "Victor" wheel, one finds that a single 54" 
wheel would do the work, but at the inconveniently low speed 
of 128 r.p.m. But under the same head a pair of 39'' wheels 




Fig. 209. 



would give a little larger margin of power at 180 r.p.m., and 
hence would probably enable one to get his dynamo at lower 
cost, as well as to avoid a thrust bearing. Often such a change 
of plan will allow the use of a standard generator where a 
special one would otherwise be necessary. 

Wherever possible it is highly desirable to employ these hori- 
zontal wheels for electrical purposes, inasmuch as power has, in 
most cases, to be transferred to a horizontal axis, and the use 
of a vertical shaft wheel necessitates some complication and 
loss of power in changing the direction of the motion. Occa- 
sionally a vertical shaft wheel is used for electrical purposes, 
driving a dynamo having a vertical armature shaft. This prac- 



360 ELECTRIC TRANSMISSION OF POWER. 

tice is not generally to be recommended, as it involves special 
dynamos, and a somewhat troublesome mechanical problem 
in supporting the weight of the armature, which is generally 
carried by hydraulic pressure. A fine example of this arrange- 
ment is to be found in the great Niagara Falls plant. 

The use of pressure turbines for driving electrical machinery 
is exceedingly convenient on low or moderate heads, say up to 
50 or 100 feet. With higher heads frequently the rotative speed 
becomes inconveniently great; for example, under 100 feet 
head, 150 HP can be obtained from a wheel a little more than 
15 inches in diameter, at a speed of more than 1,000 revolu- 
tions per minute. At 200 feet head, the power for the same 
wheel will have risen to about 400 HP and the speed to nearly 
1,300. This is a rather inconvenient speed for so large a power, 
and it is necessitated by the fact that a pressure turbine to 
work under its best conditions as to efficiency, must run at 
a peripheral speed of very nearly three-quarters the full 
velocity of water due to the head in question. If, therefore, 
turbines are used for high heads, either the dynamos to 
which they are coupled must be of decidedly abnormal design, 
or the dynamo must be run at less speed than the wheels. 

The former horn of the dilemma was taken in the Niagara 
plant, and- involved some very embarrassing mechanical ques- 
tions in the construction of the dynamos. Where belts are 
permissible the other practice is the more usual, of which a 
good example is found in the large lighting plant at Spokane 
Falls, Wash., where the wheels were belted to the dynamos 
for a reduction in speed instead of an increase, as is usually 
the case. 

Impulse turbines are little used, although manufactured 
to some extent by the Girard Water Wheel Co., of San Fran- 
cisco, Cal. The wheel manufactured by them is one with a well- 
known foreign reputation. Its general arrangement is well 
shown by the diagram. Fig. 210. The Girard impulse turbine is 
of the outward flow type, a form rather rare in pressure tur- 
bines. The water enters the wheel centrally through a set of 
guide blades, which form a series of nozzles from which the 
water issues with its full spouting velocity and impinges on the 
buckets of the runner, which surrounds the guide blades. 



WATER-WHEELS. 361 

The discharge is virtually radially outward. Regulation is 
secured by a governor which either cuts off one or more of the 
nozzles or may be arranged by swinging guide blades to con- 
tract all or a part of the nozzles. In either case, there is no 
water wasted, and the wheel works efficiently at practically all 
loads. 

Like others of the impulse type, the peripheral speed of the 
wheel when worked under its best conditions for efficiency, is 
very nearly one-half the spouting velocity of the water as it 




Fig. 210. 

issues from the nozzle. This produces for a wheel of given 
diameter a lower speed for the same head than in the case of 
pressure turbines, while the use of a larger number of nozzles 
working simultaneously on the runner gives a higher power for 
the same diameter than in the case of the Pelton or similar 
wheels, which use only a few nozzles with jets applied tan- 
gen tially; hence, such impulse turbines occupy a useful place 
in the matter of speed, aside from all questions of efficiency. 

Under moderately high heads, from 100 up to 300 or 400 
feet, they give a much greater power for a given rotative 



362 ELECTRIC TRANSMISSION OF POWER. 

speed than impulse wheels employing only two or three 
nozzles. On the other hand they do not run inconveniently 
fast, as is the case with pressure turbines under such heads. 
At extremely high heads they give, unless operated with 
only one or a few nozzles, so great power as to be inconve- 
nient for the high speed attained, so far at least as the oper- 
ation of dynamos is concerned. At very low heads there 
is material loss from the fact that the wheel cannot be used 
with the draft tube, and consequently a certain amount of the 
head must be sacrificed to secure free space from the wheel to 
the tail-water. These Girard turbines are made with both 
vertical and horizontal axes, and are applicable to electrical 
work with the same general facility which applies to other 
types of wheel. Their strong point is economical and effi- 
cient regulation of the water supply, together with high effi- 
ciency at moderate loads. 

The Pelton wheel, already shown in Fig. 202, may be regarded 
as an impulse turbine having a single nozzle, and that applied 
tangentially. These wheels have proved immensely effective 
for heads from several hundred up to a couple of thousand feet. 
Like the true impulse turbines, the peripheral speed should be 
half the spouting velocity of the water, hence, by varying the 
dimensions of the wheel a wide range of speed can be obtained, 
which is exceedingly convenient in power transmission work, 
permitting direct couphng of the dynamos under all sorts of 
conditions. They are not infrequently made with two or three 
nozzles, which give, of course, correspondingly greater power 
for the same speed. At heads of only 100 or 200 feet these 
wheels with their few nozzles give an inconveniently low rota- 
tive speed for the power developed, and are at their best in 
this respect between 300 and 1,000 feet. The Pelton wheel 
is usually regulated by deflecting the nozzles away from the 
buckets of the wheel, a very effective but most inefficient 
method, so far as economy of water is concerned. The wheel 
has, however, under favorable conditions, a very high efficiency, 
certainly as high as can be reached with any other form of 
hydraulic prime mover. The practical results given by this 
class of wheel are admirable under circumstances favorable to 
their use, and the Pelton and Doble wheels have played a very 



WATER-WHEELS. 863 

important part in the great power transmission works which 
have placed the Pacific coast in the van of modern engineering. 

A recent improvement in impulse wheel practice is the 
development of a successful ''needle valve" for the nozzles, 
which obviates the waste of water due to the use of deflecting 
nozzles. The needle valve is simply a nozzle which can be 
closed at will by a central plunger, moving axially in the 
stream just behind the nozzle. The plunger and its seat are 
given surfaces curved in such wise that in all positions of the 
plunger a smooth emergent stream is produced, and the effi- 
ciency of the wheel is very little changed. 

This is upon the whole, a better method of regulation than 
the deflecting nozzle in that it is economical of water, but 
shutting off the stream quickly produces very severe strains 
in the pipe line and in most instances some form of relief valve 
is desirable to reduce the pressure. To use any form of nozzle 
valve, too, the water must be thoroughly freed from sand, 
which at the stream velocities often used, cuts even the toughest 
metal with great rapidity. 

Another wheel of this class is the Leffel ''Cascade" water- 
wheel. Two complete rings of buckets are employed for this 
wheel, and the wheels are arranged to be supplied from several 
nozzles, of which one or more are put into use according to 
the necessities of regulation. The cascade wheel therefore 
occupies a place, as it were, between the ordinary impulse 
wheel and the impulse turbine, resembling the former in the 
arrangement of its multiple jets, and the latter in the method 
of regulation by cutting off completely some of the nozzles. 

From the foregoing it will be appreciated that each of the 
three general classes of wheels described, pressure turbines, 
impulse turbines, and tangential impulse wheels, has a sphere 
of usefulness in which it can hardly be approached by either 
of the others. It is worth while, therefore, to examine some- 
what in detail the conditions of economy under various cir- 
cumstances. 

The pressure turbine has its best field under relatively low 
and uniform heads. By means of the draft tube no head is 
lost, as is the case with that portion of the head which lies 
betweeA the turbine and the tail-water in the use of impulse 



364 



ELECTRIC TRANSMISSION OF POWER. 



wheels of any description. Further, the pressure turbine under 
all heads gives a higher relative speed than the impulse wheels, 
whether of the tangential or turbine variety, and under low 
heads is apt to be of less bulk and cost and to give a more con- 
venient speed for electric work; hence, these pressure turbines 
have been more extensively used than any other variety of 
water-wheels in the enormous hydraulic developments of the 
last quarter of a century. Furthermore, the pressure turbine 
has, under favorable conditions, as high efficiency as any known 

100 



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5 80 

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PROPORTIONAL DISCHARGE 
Fig. 211. 

variety of water-wheel. The losses of energy are mainly of 
four kinds. 

1. Friction of bearings, usually small. 

2. Friction and eddying in the wheel and guide passages. 

3. Leakage, and 

4. Unutilized energy of other kinds, largely owing to imper- 
fect shaping of the working parts, or loss of head. 

With the best construction these losses aggregate 15 to 
20 per cent. Of them the shaft friction is the smallest and 
the loss from friction and eddies in the wheel the largest, 
probably fully half of the total loss, particularly under high 



WATER-WHEELS. 365 

heads. This efficiency is approximately true of the better 
class of turbines, whether of the pressure or impulse variety. 
Under low and uniform heads the pressure turbine probably is 
capable of a little better work than the impulse variety, but both 
suffer if the head varies. The curves. Fig. 211, show efficiency 
tests made with the greatest care on four first-class pressure tur- 
bines at the Holyoke testing flume, probably the best equipped 
place in the world for making such tests. It should be noted 
that the efficiency of all the wheels shown is good; over 80 
per cent at full admission of water; at partial admission the 
efficiencies vary more between the individual wheels. This 
variation is largely due to the methods of regulating the flow 
employed. These are in general three: 

1. Varying the number of guide passages in use. 

2. Varying the area of these guide passages by moving the 
guide blades. 

3. Varying the admission to the guide passages by a gate 
covering the entrance to all of them. 

The first method is particularly bad, as the buckets are at one 
moment exposed to full water pressure and then come opposite 
a closed passage, setting up a good deal of unnecessary shock 
and eddying. It is a method that is scarcely ever used in this 
country. Between the other two it is not so easy to choose. 
Both have strong advocates among wheel makers; some com- 
panies building both types, and the others only one of them. 
The curves shown represent both these methods of regulation. 

The truth probably is that the relative efficiency of the two 
depends more on the design of the wheel with reference to its 
particular form of regulation, than on the intrinsic advantages 
of either form. Turbines are generally constructed so that 
the point of maximum efficiency is rather below the maxi- 
mum output, as a little leeway is desired for purposes of regu- 
lation under varying heads, so that the design is arranged to 
give the best efficiency of which the wheel is capable at a point 
a little below full admission. These efficiency curves were 
taken at heads of from 15 to 18 feet and show what can be regu- 
larly accomplished by good wheel design. They are neither 
phenomenally high nor unusually low. Occasionally efficiencies 
are recorded slightly better than those shown. In this connec- 



366 ELECTRIC TRANSMISSION OF POWER. 

tion it is desirable to state in the way of warning, that there 
was obtained at the Holyoke flume some years ago a series 
of tests of turbines of more than one make, which showed 
enormously high efficiency, afterward traced to a constant 
error in the experiments. As the fact of the error was not 
so generally known as the result of the tests, occasionally 
reports are heard of phenomenal turbine efficiencies which 
are given in entire good faith, but based on errors of experi- 
ment. It is only fair to say that the tests now made at the 
Holyoke flume are worthy of entire confidence. 

As regards impulse turbines, data are hard to obtain. 
Those which are available indicate, however, that with an effi- 
ciency probably a little less at full load than that of pressure 
turbines under moderate heads, the half-load efficiency is 
generally considerably higher. This is owing to the fact that 
the buckets of the runner work entirely independently of each 
other, and the water acts in precisely the same way on each 
bucket whether it is received from all the nozzles formed by the 
guide blades, or from a part of them. The impulse turbines are 
generally regulated by cutting off more or less of the nozzles. 
The shaping of the surfaces in the runner and guide blades, 
and the smoothness of the finish, are of more importance 
in these wheels than in the ordinary pressure turbines. The 
impulse turbines are, as has already been stated, peculiarly 
adapted in point of speed and general characteristics for use 
on moderately high heads, and in this work they give a better 
average efficiency and more economical use of water than any 
of the pressure turbines. For low heads their advantages are 
far less marked, and the pressure turbines are generally 
preferred. 

The tangential impulse w^heels are, at full admission of water, 
of an efficiency quite equal to that of the best turbines. At 
partial admission they cannot be expected to give the same 
results as do the best impulse turbines, inasmuch as they regu- 
late generally by deflecting the nozzle away from the buckets, 
and hence wasting water. The variation of the stream by a 
needle valve considerably relieves this difficulty in cases where 
it can be successfully appHed. For very high heads, however, 
the tangential wheels are preferable to any turbines, as they 



WATER-WHEELS. 367 

give a better relation between power and speed, so far as driv- 
ing electrical machinery is concerned, and their extreme 
simplicity is favorable to good continuous working under the 
enormous strains produced by the impact of water at great 
spouting velocities. 

To summarize, pressure turbines are admirable for low and 
uniform heads, particularly where the load is steady. The 
impulse turbines give more efficient use of water at part load, 
and a more convenient speed on moderately high heads. The 
tangential impulse wheels do relatively the best work under 
very high heads, and where water does not have to be rigidly 
economized. Each of the three classes has decided advantages 
over the others in particular situations, and the full load effi- 
ciency of all three is approximately equal. The choice of 
either one of these types should be made in each individual 
case in accordance with the hydraulic conditions which are to 
be met. The choice between particular forms of each type is 
largely a commercial matter, in which price, guarantees, 
facility of getting at the makers in case of repairs, standard 
sizes fitting the particular case in hand, and similar considera- 
tions are likely to determine the particular make employed, 
rather than any broad difference in construction or operation. 

The success of a power transmission plant depends quite as 
much on careful hydraulic work as on proper electrical instal- 
lation. The two should go hand in hand, and any attempt, 
such as is often made, to contract for the two parts of the 
plant independently of each other, or to engineer them inde- 
pendently, generally results in a combination of electrical and 
hydraulic machinery that is far from being the best possible 
under the conditions, and is quite likely to be anything but 
satisfactory. 

The hydraulic and electrical engineers should go over 
the arrangement of the plant together with a view to adapting 
each class of machinery to the other as perfectly as possi- 
ble, in order to get a symmetrical whole. Many troublesome 
questions have to be encountered, and only the closest study 
will lead to perfectly successful results. 

One of the commonest and most serious difficulties met with 
in laying out an electrical and hydraulic plant for transmission 



368 ELECTRIC TRANSMISSION OF POWER. 

work, lies in the variability of the head of water. There are 
comparatively few streams from which can be obtained an in- 
variable head practically independent of low water or freshets. 
The usual condition of things is to find a fairly uniform head 
for nine or ten months in the year, and rather wide variations 
during the remainder of the time. It is not at all uncommon to 
meet a water-power which, even when very skilfully developed, 
will still entail upon the user a variation of 25 or 30 per cent 
in the available head. 

At the time of high water this appears as a rise of water 
level in the tail-race, so as to diminish the head available for the 
wheels. In times of low water, the head might be normal, but 
the quantity of water altogether insufficient. Any variations 
of this kind are of a very serious character, because they not 
only vary the amount of power which is available, but they 
change the speed of the wheels so that the dynamos no longer 
will operate at their proper speed and hence will change in 
voltage, and if alternating apparatus is used, in frequency also, 
which is even more serious. For example: Under 24 feet 
head one of the well-known standard wheels gives nearly 
650 HP at 100 revolutions per minute. Under 16 feet head 
the same wheel would give only 352 HP at 82 revolutions. 

The lack of power occurring at the time of high water is 
serious. The change of speed, although not great, is very 
annoying, and should be avoided if possible. Changes much 
greater than this are common enough. The season of reduced 
head is generally short, not over a couple of months, often 
only a week or two, and this renders the situation doubly 
embarrassing, because during a large part of the year the same 
wheel must be able to operate economically. The methods 
taken to get out of this difficulty of varying head are various; 
most of them bad. One of the commonest is to arrange the 
wheels to operate normally at partial gate, then on the low 
heads to throw the gate wide open and obtain increased power. 
On the high heads the wheel is throttled still more. Such an 
arrangement works the dynamo in a fairly efficient fashion, 
but the wheel, as a rule, quite inefficiently a large portion of 
the time, as may be seen by reference to the efficiency curves 
of the wheels just given. It is a practice similar to that which 



WA TER-W HEELS. 369 

one would find in working an engine at part load. For moder- 
ate variations of head, not exceeding 10 per cent, the loss of 
efficiency is not so serious as to bar this very simple plan, but 
under conditions too frequently encountered, these variations 
of efficiency would be so great as to make the method exceed- 
ingly undesirable. 

Hydraulic plants are occasionally operated without any 
reference to economy of water, and in such cases the practice 
of operating normally at part gate is frequently followed, 
but it must be remembered that as water powers are more 
and more developed, economy of water becomes more and 
more necessary, and in every case should be borne in mind 
even if it is not rendered necessary by conditions actually 
existing. In thoroughly developed streams it is generally 
important to waste no water. 

Another method of overcoming the difficulties due to 
variations of head, is the installation of two wheels on the 
same shaft, one intended to give normal power and speed at 
the ordinary head, the other at the emergency head. This 
practice is carried out in various forms. Sometimes two 
wheels may be mounted on the same horizontal or vertical 
axis, and one of them is disconnected or permitted to run 
idle except when actually needed. Another modification of 
the same general idea is the use of a duplex wheel with the 
runner and guides arranged in two or three concentric sets of 
buckets, which can be used singly or together according to 
the head which is available. 

A fine example of this practice is found in the great power 
plant at Geneva, to which reference has already been made, 
where the head varies from 5^ to 12 feet. Here the turbines 
have buckets arranged in three concentric rings, the outermost 
being used at the highest head and all three at the lowest head. 
Under the latter condition, the average radius at which the 
water acts upon the wheel is diminished and the speed is 
therefore increased, while the greater volume of water keeps 
up the power. The various combinations possible with the 
rings of buckets are so effective in keeping the speed uniform 
that the extreme variation of speed under the maximum varia- 
tion of head is only about 10 per cent. Such a triplex turbine 



370 ELECTRIC TRANSMISSION OF POWER. 

is of high first cost, but is decidedly economical of water at 
normal load. Still another variation of the double turbine 
idea consists of installing two turbines for each unit of power, 
one acting directly, the second through the medium of belts. 
The direct-acting turbine is intended for normal load, the 
belted turbine of larger dimensions for use during the periods 
of low head. 

This arrangement is used in the large power transmission 
plant at Oregon City, Ore. It is economical of water, but is 
mechanically somewhat complicated. It is probably on the 
whole less desirable than the installation of two turbines on 
the same shaft, and much less desirable than the duplex or 
triplex arrangement just referred to. Where two turbines are 
operated on the same shaft, it is generally possible to arrange 
the turbine designed to operate on the lower head so as to run 
at a disproportionately high velocity with some loss of effi- 
ciency, and so to hold the speed fairly uniform. 

Still another method of counteracting the variation of 
head is applicable only where the power is transmitted from 
the turbine by gears or belts. In this case it is always possi- 
ble to operate the machinery under the reduced head with 
some loss of output, but still at or near the proper speed. 
Whatever way out of the difficulty is chosen, it should be 
borne in mind that the most desirable, on the whole, is the 
one which will work the wheels during the generally long 
period of fairly steady head at their best efficiency. If there 
is to be any sacrifice of efficiency, it should by all means be 
for as short a time as possible, and, therefore, should be at 
the periods of extreme low head. At such times water is 
generally plenty, while at the higher heads economy in its 
use is more necessary. 

REGULATION OF WATER-WHEELS. 

For many years there have been bad water-wheel governors 
and worse water-wheel governors, but only recently have there 
appeared governors which may be classified as good from the 
standpoint of the electrical engineer. It has been necessary 
to go through the same tedious period of waiting and experi- 



WATER-WHEELS. 371 

mentation that was encountered before dynamo builders could 
find engines which would hold their speed at varying loads. 
Until the advent of electrical transmission work, water-wheels 
were most generally employed for certain classes of manufac- 
turing, such as textile mills, where the speed must be quite 
uniform, but where at the same time the load is almost uniform; 
or, on the other hand, for saw mills and the like, where constant 
speed is of no particular importance. 

The action of water-wheel governors as regards the way in 
which they vary the supply of water is very different; some 
merely act to open or close the head gates; others to work a 
cylinder gate immediately around the wheel, and still others 
to vary the area of the guide passages, as in the so-called 
register gate turbines. 

In whatever way the governing action takes place, its 
result is too often unsatisfactory, due to the great difficulty 
that has to be encountered in the great inertia of the water 
and of the moving parts of the wheel. Both water and wheel 
are sluggish in their action, and as a result some time elapses 
after the governor has produced a change of gate, before 
that change becomes effective. Meanwhile, the speed has 
fallen or risen to a very considerable extent, and perhaps in 
addition the load has again changed so that by the time the 
speed of the wheel has been sensibly affected by the governor, 
the direction of the governing action may be exactly opposite 
to that which at the moment is desirable. Even if this is not 
the case, the governing is usually carried too far, being con- 
tinued up to the time at which the wheel is affected and reacts 
on the governing apparatus, hence another motion of the 
governor becomes necessary to counteract the excess of dili- 
gence on the part of the first action. In other words, the 
governor ''hunts," causing a slow oscillation of the speed 
about the desired point, an oscillation of decreasing amplitude 
only if the new load on the wheel be steady. 

This sluggishness of reaction to changes indicated by the 
governor is the most formidable obstacle to the proper control 
of the water-wheels. To overcome it, even in part, it is 
necessary that the movement of the gates be comparatively 
active, if the changes of load are frequent, and this entails still 



372 ELECTRIC TRANSMISSION OF POWER. 

further difficulty by causing severe strains on the mechanism 
and the gates, particularly if the water is led to the turbine 
through a long penstock. In the latter case, the variations in 
pressure produced by rapid governing are often dangerous, 
and have to be counteracted by air chambers, stand pipes, or 
the like, and aside from all this there is a still further difficulty 
in the considerable weight of the gate and the pressure against 
which they have to be operated, so that the amount of mechani- 
cal power controlled by the governor must be very consider- 
able. 

A very large variety of governors have been designed to 
meet the serious difficulties just set forth. Most of them 
have been abject failures, aud those that may be really reck- 
oned of some considerable value for electrical work may be 
counted on the fingers of one hand. 

Water-wheel governors may be roughly divided into two 
classes. First, come those regulators in which the wheel itself 
supplies power to the gate-shifting mechanism, which is con- 
trolled by a fly ball governor through more or less direct 
mechanical means. Second, comes the relay class of governors, 
wherein all the work possible is taken off the centrifugal 
governor, and its function is reduced to throwing into action 
a mechanism for moving the gates which may be quite inde- 
pendent of any power transmitted from the wheel to the gov- 
erning mechanism. The various classes of hydraulic, pneu- 
matic, and electric governors are worked in this way. Their 
general characteristic is that their sole function in governing 
is to work the devices which control the secondary mechanism, 
which consists, in various cases, of hydraulic cylinders oper- 
ating the gates, pneumatic cylinders serving the same pur- 
pose, or electric motors which open or close the gates by 
power derived from the machines operated by the turbines. 

A vast amount of ingenuity has been spent in trying to 
work out regulators of the first mentioned class. Almost 
every possible variety of mechanism has been employed 
to enable the governor to apply the necessary power to the 
mechanism operating the gates. The general form of most of 
these governors is as follows: Power is taken from the wheel 
shaft by a belt to the governor mechanism, where it serves at 



WA TER-WHEELS. 878 

once to drive the governor balls, and to work the gates when 
the governor connects the gate-controlling gears to the pulley 
which supplies the power. This is generally done by friction 
cones or their equivalent, thrown into action in one direction 
when the governor balls rise, and in the other direction when 
they fall. 

Sometimes this mechanism is varied by employing a pair 
of oscillating dogs, one or the other of which is thrown 
into appropriate gearing by the governors. There are many 
governors of this kind on the market, and where the load is 
fairly steady and no particular accuracy of regulation is neces- 
sary, they have given good satisfaction. The fault with all 
governors of this sort is that the centrifugal balls either lack 
sensitiveness or lack power. If the governor works at all 
rapidly in moving the gates, too heavy a load is thrown on 
the governor for any but a massive mechanism, and the cen- 
trifugal device becomes insensitive; or, on the other hand, if 
the gates are worked slowly, the governor in itself is sluggish 
and ineffectual. 

In most cases the gates are made to move quite slowly. 
In the attempt to get sensitiveness, the friction wheels or 
dogs are often adjusted so closely that the governor is in a 
constant sHght oscillatory motion, but when its action is 
really needed, as in the case of a sudden change of load, 
response generally does not come quickly enough. It is of 
course possible to construct a mechanical relay which would 
possess both power and sensitiveness, but nearly all the 
governors made on this principle lack one or the other, and 
sometimes both. 

The second type of governor, as mentioned, is not open to the 
objections noted, if properly designed, inasmuch as it is a 
comparatively easy matter to make a balanced hydraulic or 
pneumatic valve which can be worked even by the most sensi- 
tive of governors, and yet can apply power enough to move 
heavy gates as rapidly as is consistent with safety. In ad- 
dition, such governors can be made to work with a rapidity 
depending on the amount of change in speed, so that if a 
heavy load is thrown on the wheel, the relay valve* would 
be thrown wide open, and consequently bring a great and 



374 ELECTRIC TRANSMISSION OF POWER. 

immediate pressure to bear upon the gates. In the so-called 
electric governors, the function of the governor balls is merely 
to make in one direction or the other the electrical connec- 
tions to a reversible motor which handles the gates. This 
relay class of governors has been recently worked out with 
considerable care, and is capable of giving surprisingly close 
regulation even under widely varying loads, results compar- 
able even with those obtained from a steam engine governor. 

A third, type of water-wheel governor is independent of 
any centrifugal device and operates by a differential speed 
mechanism, so that wherever the speed of the wheel varies from 
a certain fixed speed maintained by an independent motor, 
the gates are opened or closed as occasion demands. The 
difficulty here is to get a constant speed which will not be 
sensibly altered when the load of working the gates is thrown 
on the governor mechanism. Some species of relay device 
is almost necessary to the successful operation of a differential 
governor, but with such an adjunct very close regulation 
can be and is obtained. 

Up to the past few years almost all hydrauHc governing has 
been by mechanisms of the first class, and it is only recently 
that the relay idea has been worked out carefully, both for 
centrifugal and differential mechanisms, so as to obtain any- 
thing like satisfactory results for electrical work where close 
regulation of speed over a wide variation of load is very 
necessary. 

For electrical purposes, several rather interesting governing 
mechanisms have been tried, which do not fall into any of these 
classes, inasmuch as their function is to keep the load con- 
stant and prevent variations of speed instead of checking these 
variations after they have been set up. Such governors (load 
governors they may properly be called) operate by electric 
means, throwing into circuit a heavy rheostat or a storage 
battery when the electrical load falls off, and cutting these 
devices out again when the load in the main circuit increases. 
These governors have in several instances been applied 
with success to controlhng the variable loads found in electric 
railway* stations operated by water-wheels. But they waste 
energy in a very objectionable manner, and at best can only 



WATER-WHEELS. 375 

be regarded as bad makeshifts, out of the question when there 
must be any regard for economy of water, and only to be 
tolerated in the lack of an efficient speed regulator. 

Occasionally electric governors operated by the variations 
in the voltage of the circuit supplied have been tried, but 
these are open to two serious objections: In the first place, 
they do not hold the voltage steady for the same reasons that 
most speed regulators do not hold the speed steady. Secondly, 
they regulate the wrong thing. In transmission plants, most 
of which are and will be operated by alternating currents, it 




Fig. 212. 

.is important that the frequency be kept imiform. If the vol- 
tage is kept constant by varying the speed, the frequency is 
subject to enough variation to be very annoying in the opera- 
tion of motors. Automatic voltage regulators, working through 
variation of the field excitation of the generator, belong in a 
different category and have come into considerable and suc- 
cessful use. 

To pass from the general to the special. Fig. 212 shows a 
typical water-wheel governor of the first class, that is, of the 
kind operated directly by the wheel through a system of dogs 
worked by a fly ball governor. There is here no attempt at 
delicate relay work, and the resulting mechanism, while quite 
good enough for rough-and-ready work, is of little use for 
any case where a variable load must be held to its speed with 
even a fair degree of accuracy. The cut shows the construction 



376 ELECTRIC TRANSMISSION OF POWER, 

well enough to render further description superfluous. Gov- 
ernors like these were practically the best available for many 
years, and proved to be cheap and durable, but they seldom 
governed much more than to keep the wheels from racing 
dangerously when the load was thrown off, or from slowing 
down permanently when it came on. It is not too much to 
say that they never should be used in connection with an 
electrical station, unless combined with intelligent hand regu- 
lation — which at a pinch is not to be despised. 

Of the indirect acting and relay governors there are many 
species, most of which had better be consigned to the oblivion 
of the scrap heap. But out of the manifold inventions and 
experiments good has come, so that at the present time there 
are a few delicate relay governors capable of holding the wheel 
speed constant within a very narrow margin indeed. Others 
of similar excellence will probably be evolved, but just now 
three, the Lombard, Replogle, and the Faesch-Piccard, together 
with one or two electrical governors, are decidedly the best 
known. The first named has given very remarkable results 
in many transmission plants in which it has been employed — 
results quite comparable with those obtained from a well-gov- 
erned steam engine. The second has given excellent results 
in the Oregon City transmission and elsewhere, while the last 
was adopted for the original transmission at Niagara Falls 
and has done its work well, although in the extension of the . 
plant an hydraulic relay governor designed by Escher, Wyss 
& Co., was installed. They are suitable types of the hydrauHc, 
electric,' and mechanical relay governors. 

The Lombard governor, Plate XIII, is an hydraulic relay in 
principle. The gate-actuating mechanism is a rack gearing 
into a pinion, and driven to and fro by the piston of a pressure 
cylinder. The working fluid is thin oil, kept under a pressure 
of about 200 lbs. per square inch. This pressure is suppHed 
by a pump driven by the pulley shown in the figure and 
operating to keep up a 200 lb. air pressure in the pressure 
chamber at the base of the governor, above the oil that par- 
tially fills it. This chamber is divided into two sections, the 
one holding the oil under pressure, the other being a vacuum 
space kept at reduced pressure by the pump system. 



WATER-WHEELS. 



377 



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378 ELECTRIC TRANSMISSION OF POWER. 

The circulation of oil is from the pressure chamber through 
the piping system and valves to the working cylinder, and 
thence into the vacuum chamber, whence it is pumped back 
into the pressure chamber again. The governor proper con- 
sists of a sensitive pair of fly balls operating a balanced piston 
valve in the path of the pressure oil. A motion of ^^ of an 
inch at the valve is sufficient to put the piston into full action 
and open or close the gates. Sensitive as this mechanism is, it 
would not govern properly without the addition of an ingenious 
device, peculiar to this governor, to take account of the inertia 
of the system. The weakest point of all such governing 
mechanisms has been their helplessness in the matter of inertia. 
If a governor even of the sensitive relay class be set to regu- 
late a wheel, we encounter the following unpleasant dilemma: 
If the mechanism moves the gates quite slowly, it will not be 
able to follow the changes of load. If it moves them rapidly 
the governing overruns on account of the inertia of the whole 
wheel system, so that the apparatus "hunts," perhaps the 
worst vice a governor can have when dynamos are to be gov- 
erned. Hence most governors have either been unable to 
follow a quickly varying load at all, or they have made matters 
worse by hunting. 

In the Lombard governor, special means are provided to 
obviate hunting. The bell-crank lever seen in the background 
of Plate XIII is actuated by the same movement that works the 
wheel gates, and moves the governor valve independently of 
the fly balls. Its office is promptly to close the valve far enough 
ahead of the termination of the regular gate movement to 
compensate for inertia. For example, if the speed falls and 
the fly balls operate to open the gate wider, the lever in ques- 
tion closes the governor valve before the fly balls are quite 
back to speed, so that instead of overrunning and hunting, the 
governing is practically dead beat. 

The result obtained with this governor is well seen in Fig. 213. 
This diagram is taken from a plant operating an electric street 
railway — perhaps the worst possible load in point of irregu- 
larity. The diagram shows a maximum variation of 2.1 per 
cent from normal speed, lasting less than one minute, under 
extreme variation of load. These results are entirely authen- 




PLATE XIII. 



WA TER-W HEELS. 379 

tic, the readings having been taken jointly by the represen- 
tatives of the governor company and the local company. Speed 
was taken by direct reading tachometer and load from the 
station instruments. 

Fig. 214 shows a small governor of the same make intended 
for use with impulse wheels, and for similar light work under high 




i?lG 214. 

heads. It works on precisely the same principle as the larger 
governor, save that the power is derived from a water cylinder 
taking water from the full head of the plant. The work of 
this Httle governor is 5.4 foot-pounds for each foot of working 
head, quite enough to handle the deflecting nozzles or needle 
valves used for regulating impulse wheels. The larger governor 
of Plate XIII has a very different task in moving the heavy 



380 ELECTRIC TRANSMISSION OF POWER. 

gates of turbine wheels and is designed to develop more than 
10,000 foot-pounds. For still heavier duty a vertical cylinder 
type of Lombard governor, somewhat simplified from the 
forms here shown, has recently been introduced. 

A gate gauge is generally attached to the bed plate of the 
Lombard governor, so that the excursions of the piston plainly 
show the exact extent of gate opening. The mechanism of 
the governor is decidedly complicated, but it is extremely 
well made and fitted, so that it seldom gets out of order. It 
permits readily of all sorts of adjustment with respect to the 
speed, but for power transmission work one needs constant 
speed only, except when varying speed temporarily in syn- 
chronizing a generator. The invariable rule, therefore, should 
be to adjust the governor carefully for the exact speed required, 
and thereafter to let its adjustments alone as long as it 
continues to hold that speed. In power transmission work and 
in railway plants, this governor -is at present used probably 
more than all others combined. 

The Faesch & Piccard governor has taken several forms, 
the idea of a sensitive relay mechanism being carried through 
all of them. An hydraulic relay has been successfully em- 
ployed abroad. In this the function of the fly balls is reduced 
to moving a balanced valve controlling hydraulic power 
derived from the natural head, or from a pressure cylinder. 
There is no mechanical provision against hunting, but the 
speed of governing is adjusted as nearly as possible to the re- 
quirements of the load, and the results are generally good. In 
the great Niagara plant the governor is situated on the floor 
of the power house, nearly 140 feet above the wheel. It is a 
very sensitive mechanical relay, in which the motion of a pair 
of fast running fly balls puts into operation through a system of 
oscillating dogs a brake-tightening mechanism, which in its turn 
permits power to be transferred from pulleys driven from the 
turbine shaft through a pair of dynamometer gears, to the system 
of gearing that works the balanced gate at the end of the lever 
system 140 feet below the governor. This governor was guar- 
anteed to hold the speed constant within 2 per cent under ordi- 
nary changes of load, and to limit the speed variation to 4 per 



WATER-WHEELS. 



381 




4 D r— 3 > 



382 ELECTRIC TRANSMISSION OF POWER. 

cent for a sudden change of 25 per cent in the load. Fig. 215 
gives a good notion of the principles of this apparatus, which 
is fairly satisfactory. The Replogle governor is an electro- 
mechanical relay shown in Fig. 216, whch exhibits its general 
arrangement very well. The work done by the fly balls is 
very trifling and the mechanism is both sensitive and powerful. 




Fig. 216. 

Fig. 217 shows its performance in governing a railway load 
under conditions of unusual severity. As in Fig. 213, 20 min- 
utes of operation are plotted and the maximum variation from 
105 revolutions per minute, the normal speed, is less than 10 
revolutions, and that variation lasted less than 20 seconds and 
was due to the opening of the circuit-breaker. Such work is 
quite good enough to meet all ordinary conditions. 



WATER-WHEELS. 



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ELECTRIC TRANSMISSION OF POWER. 



To a very different type of mechanism belongs the differen- 
tial governor shown in Fig. 218. It has been appHed widely to 
the governing of Pel ton impulse wheels, with very excellent 
results. The principle involved is very simple. Two bevel 
gears, each carrying on its shaft a pulley, are connected by a 
pair of bevel gears on a crosswise shaft, forming a species of 
dynamometer gearing. Normally the main gears are driven 
in opposite directions, the one at a constant speed by a special 




Fig. 218. 



source of power, the other from the shaft to be governed. 
So long as the speeds of these wheels are exactly equal and 
opposite, the transverse shaft remains stationary in space and 
the gate moving mechanism attached to it is at rest. When, 
however, the working shaft changes speed under the influence 
of a change in load, the transverse shaft necessarily moves in 
one direction or the other and keeps on moving until the 
working shaft gets back to speed. 

In practice the main difficulty is to hold the constant speed 
necessary for one of the bevel gears, and the governor works 
admirably or badly as this constancy is or is not maintained. 



WATER-WHEELS. 385 

A heavy fly wheel on the constant speed side is desirable, and 
its motive power should be quite independent of the main 
drive. Perhaps the best result is obtained by using a second, 
small, differential governor to hold the speed uniform at the 
main governor. With the high heads and balanced deflecting 
nozzles usual in Pelton wheel practice, this form of governor 
is very sensitive and does not hunt noticeably, owing to the 
small inertia of the moving parts. It gives good regulation 
under all conditions except extreme variations of load where 
the wheel is loaded beyond the power of the jet to enforce 
prompt recovery of speed, and is well suited to the conditions 
under which Pelton wheels are generally used. 

The greatest difficulty in hydraulic governing is that of 
hydrauHc inertia. Water moves sluggishly through long and 
level pipes, and its velocity does not change promptly enough 
for good governing, unless the waterways are planned with 
that in view. If a wheel is at the end of a long and gently 
sloping penstock it takes a certain definite amomit of time to 
get that water column under way or checked in response to 
the movement of the gates. And the longer this time con- 
stant of the water column the more difficult it is to get accu- 
rate governing, however good the governing mechanism may 
be. For by the time the water gets fairly into action the load 
conditions may have changed, and the governor may be again 
actively at work trying to readjust the speed. 

In order to get accurate governing it is absolutely necessary 
to keep the time constant of the waterways as small as pos- 
sible. To accomplish this the regulating gate should obviously 
be right at the wheel and the penstock should be as short and 
as nearly vertical as possible. The most favorable condition 
for governing is when the wheel is practically in an open 
flume. If steel penstocks are used they should pitch as 
sharply down upon the wheels as conditions permit, some- 
thing after the manner of Fig. 208. If long head pipes must 
be used governing will become difficult, although much help can 
be obtained from an open vertical standpipe connected with 
the penstock close to the wheel. The contents of this pipe 
serve as a pressure column if the gate is suddenly opened and 
as a relief valve if the gate suddenly closes, averting the some- 



386 ELECTRIC TRANSMISSION OF POWER. 

times serious pressure due to the violently checked stream. 
Plate XIV shows such a standpipe in action just after a heavy 
electric load had been thrown off. The water normally stands 
very near the top of the pipe, which begins to overflow with a 
slight increase of the hydraulic pressure. 

Under high heads such a standpipe is of course impracti- 
cable, and although some forms of relief valve are of use, the 
conditions of governing are not easy until one comes to the 
impulse wheel with a deflecting nozzle. 

Not all water-wheels are governed with equal ease. If the 
gates are properly balanced a comparatively small amount of 
power will manage them promptly, and the wheel is governed 
without trouble. But there are some wheels on the market 
with gates under so much unbalanced pressure that proper 
governing is difficult or impossible. There is no excuse for 
the existence of such wheels, for they do not have compensat- 
ing advantages, and they should be shunned. All the typical 
wheels which have been described in this chapter govern 
easily, however, as do many others. It is worth while to re- 
member that good governing is absolutely indispensable for 
good service, and although one finds cases in which the load is 
so steady that the wheels can almost go without governing, 
such are rare exceptions to the general rule. 




PLATE XIY. 



CHAPTER X. 

HYDRAULIC DEVELOPMENT. 

So much electrical transmission work depends on the utiliza- 
tion of water-powers that it is worth while briefly to consider 
the subject of developing natural falls for such use. The 
subject is a large one, quite enough to fill a volume by itself, 
and the most that can be done here is to point out the salient 
facts and put the reader in possession of such information as 
will enable him to avoid serious blunders and to take up the 
subject intelligently. 

Natural water-powers of course vary enormously in their 
characteristics. In our own country, where water-power is 
very widely distributed, we find three general classes of powers, 
often running into each other but still sufficiently distinct to 
cause the methods of developing them to be quite well defined. 

By far the best known class of powers are those derived 
from the swift rivers that are found in New England and 
other regions in which the general level of the country changes 
rather rapidly. They flow through a country of rocky and 
hilly character, and large or small, are still swift, powerful 
streams, ^Adth frequent rapids and now and then a cascade. 
Such rivers are generally fed to no small extent by springs 
and lakes far up toward the mountains, and catch in addition 
the aggregated drainage of the irregular hifl country through 
which they flow. Types of this class are the Merrimac and 
the Androscoggin among the New England rivers, the upper 
Hudson, and many others. Another and quite different class 
of powers are those derived from the slow streams that flow 
through a flat or rolling alluvial country — the Mississippi 
valley and the lowlands of the Southern Atlantic States. Al- 
though possessed of many tributaries that spring from among 
the mountains, the great basins which they drain form the 
main reliance of rivers of this kind — immense areas of fertile 
country the aggregated rainfall of which supports the streams. 

387 



388 ELECTRIC TRANSMISSION OF POWER. 

Finally, there are many fine water-powers that come from 
mountain streams, fed from little springs among the rocks, 
from the melting of the winter snows and the drainage of 
heights which the snow never deserts, and from the rain 
gathered by desolate gorges. 

These mountain rivers often furnish magnificent powers, 
easy and cheap to develop, but very variable. In summer the 
stream may dwindle to a mere brook, while in spring, from 
the combined effect of rains and melting snow, it may suddenly 
increase even many thousand fold, becoming a tremendous 
torrent that no works built by man can withstand. The 
available heads are often prodigious, from a few hundred to 
more than a thousand feet, and the volume of water may seem 
at first sight absurdly small, but when, as in the Fresno (Cal.) 
plant to be described later, each cubic foot flowing per second 
means 140 mechanical HP delivered by the wheels, large 
volume is needless. 

Upland rivers like those common in New England, seldom 
give opportunity for securing high heads. Most of the powers 
developed show available falls ranging from 20 to 40 feet. 
Unless the stream has considerable volume, such low heads do 
not yield power enough to serve anything but trivial purposes — 
only two or three HP per cubic foot per second. Upland rivers, 
however, furnish the great bulk of the water-power now utilized, 
for they furnish fairly steady and cheap power under favorable 
conditions. Although subject to considerable, sometimes formi- 
dable, freshets, when the snow is melting or during heavy rains, 
they are generally controllable without serious difficulty. 

Lowland streams seldom offer anything better than very 
low heads, rarely more than 10 to 15 feet, and consequently 
demand an immense flow to produce any considerable power. 
They are, however, as a class rather reliable. The size and 
character of the drainage basin makes extremely low or ex- 
tremely high water rare, and only to be caused by very great 
extremes in the rainfall. Such streams furnish a vast number 
of very useful powers of moderate size, forming a large aggre- 
gate but seldom giving opportunity for any striking feats of 
hydraulic engineering, at least in our own country, where 
fuel is generally cheap. 



HYDRAULIC DEVELOPMENT. 389 

In taking up any hydraulic work with reference to electrical 
power transmission, or any other purpose in fact, the first 
necessary step is to make a sort of reconnoissance, to ascertain 
the general topography of the region, the available head, and 
the probable flow. The first two points are generally easy to 
determine from existing surveys or by a brief series of levels, 
the last named requires a combination of educated judgment 
and careful engineering. The U. S. Geological Survey maps 
are invaluable, when available, for getting a preliminary idea 
of the topography and the probable drainage basin. The facts 
are not really very difficult to get at, but guesswork is emphati- 
cally out of order and heresay evidence even more worthless 
than usual. The author has seen more than one mighty tor- 
rent dwindle into a trout brook when looked at through 
untinted spectacles. 

The only way to find out how much flow is available is to 
measure it carefully, if it has not already been measured in a 
thorough and trustworthy manner — not once or twice or a 
dozen times, but weekly or, better, daily, for an entire season at 
least; the more thoroughly the better. A knowledge of the 
absolute flow at one particular time is interesting, but of little 
value compared with a knowledge of the variations of flow 
from month to month, or from year to year. 

Such a series of measurements tells two very important 
things — first, the minimum flow, which represents the max- 
imum power available continuously without artificial storage 
of water; and second, the aggregate flow during any specified 
period, which shows the possibilities of eking out the water 
supply by storage. 

The methods of measurement are comparatively simple. 
For small streams the easiest way is to construct a weir across 
the stream and measure the flow over a notch of known dimen- 
sions in this weir. Such a temporary dam should be tight and 
firmly set, and high enough to back up the water into a quiet 
pool free from noticeable flow except close to the edge of the 
weir. There should be sufficient fall below the bottom of the 
notch in the weir to give a clear and free fall for the issuing 
water — say two or three times the depth of the flow over the 
weir itself. 



390 



ELECTRIC TRANSMISSION OF POWER. 



Fig. 219 shows clearly the general arrangement of a measur- 
ing weir. Here A shows the end supports of the weir, here 
composed of a single plank, while B is the lower edge of the 
notch through which the water flows. This edge B, as well as 
the sides of the notch, should be chamfered away to a rather 
sharp edge on the upstream side, which must be vertical. 
Back some feet from the weir so as to be in still water, should 
be set firmly a post E, the top of which is on exactly the same 
level as the bottom of the weir notch B. D shows this level, 
while the line C shows the level of the still water. The quan- 
tities to be exactly measured are the length of the notch B 




Fig. 219. 



and the height from the level of the edge of B to the normal 
level surface of the water in the pool. This can be done 
generally with sufficient accuracy by holding or fixing a scale 
on the top of the post E. If we call the breadth of the notch 
6, and this height h, both measured in feet, the flow in cubic 
feet per minute is 

Q ^ 40cbh \/2gh. 

Here g is 32.2 and c is the ''coefficient of contraction," which 
defines the ratio of the actual minimum area of the flowing 
jet to the nominal area h h. 

This coefficient varies shghtly with the width of the notch 



HYDRAULIC DEVELOPMENT. 



391 



as compared with the whole width of the weir dam. CaUing 
this w, the value of c is approximately 

h 
c = 0.57 + 10 -• 
w 

This gives c = .62 for a notch half the width of the weir and 
c = .67 for the full width of the weir. For notches below one- 
quarter the width of the weir the values of c become somewhat 
uncertain, and as a rule h should be over half of w. Further, 
the notch should not be so wide as to reduce the water flowing 
over it to a very thin sheet. It is best to arrange the notch so 
that the depth of water h may be anywhere a tenth to a half of 
b. For purposes of approximation weir tables are sometimes 
convenient. These give usually the flow in cubic feet per 
minute corresponding to each inch in width b, for various 
values of h. Such a table, condensed from one used by one of 
the prominent turbine makers, is given below. Where quite 
exact measurement is required the constant c should be deter- 
mined from the actual dimensions and a working table de- 
duced from it. 

Table of Weirs. 



Inches and Fractions Depth on Weir. 


9 


i 


i 


i 


1 


0.40 

1.14 

2.09 

3.22 

4.51 

5.92 

7.46 

9.]2 

10.88 

12.75 

14.71 

16.76 

18.89 

21.12 

23.42 

25.80 

28.26 

30.78 


0.56 

1.36 

2.36 

3.53 

4.85 

6.30 

7.87 

9.55 

11.34 

13.23 

15.21 

17.28 

19.44 

21.68 

24.01 

26.41 

28.88 

31.43 


0.74 

1.59 

2.64 

3.85 

5.25 

6.68 

8.28 

9.99 

11.80 

13.72 

15.72 

17.82 

20.00 

22.26 

24.60 

27.02 

29.51 

32.07 


97 


2 


1 84 


3 


2 93 


4 


4 17 


5 


5 56 


6 


7 07 


7 


8 70 


8 


10 43 


9 


12 27 


10 


14 21 


11 


16 24 


12 


18 35 


13 


20 56 


14 


22 83 


16 


25 19 


16 


27 63 


17 


30 14 


18 


32 73 







Cubic feet per minute per inch of width. 

West of the Rocky Mountains a special system of measuring 
water by ''miner's inches" has come into very extensive use. 



892 



ELECTRIC TRANSMISSION OF POWER. 



It originated in the artificial distribution of water for mining 
and irrigating purposes, and has since extended to a conven- 
tional measurement for streams. The miner's inch is a unit 
of constant flow, and varies somewhat from State to State, its 
amount being regulated by statute in various States. It is 
the flow through an aperture 1 inch square under a specified 
head, frequently 6 inches. The method of measurement is 
shown in Fig. 220. The water is led into a measuring box 
closed at the end except for an aperture controlled by a slide. 
The end board is 1^ inch thick, and the aperture is 2 inches 




Fig. 220. 



wide, its bottom is 2 inches above the bottom of the box, and its 
centre 6 inches below the level of the water. Each inch of 
length of the aperture then represents 2 miner's inches. 
Under these conditions the flow is 1.55 cubic feet per minute for 
each miner's inch. Under a 4^ inch effective head, which is 
extensively used in southern California and the adjacent 
regions, the miner's inch is about 1.2 cubic feet (9 gallons) 
per minute. 

For streams too large to be readily measured by the means 
already described, a method of approximation is applied as 
follows : 



HYDRAULIC DEVELOPMENT. 



393 



Select a place where the bed of the stream is fairly regular 
and take a set of soundings at equal intervals, a, b, c, d, Fig. 
221, perpendicular to the direction of flow, using a staff rather 
than a sounding line, as it can more easily be kept perpendicu- 
lar. Ascertain thus the area of flow. Then establish two lines 
across the stream, say 100 feet apart and nearly equidistant 
from the line of soundings. Then throw floats into the 
stream near the centre and time their passage across the two 
reference lines. This establishes the velocity of the flow 
across the measured cross section. As the water at the bottom 
and sides of the channel is somewhat retarded, the average 
velocity is generally assumed to be 80 per cent of that mea- 
sured as above in the middle of the stream. 

The more complete the data on variations of flow, the 




Fig. 221. 



better. The most important point to be fixed is the flow at 
extreme Ioav water, both in ordinary seasons and seasons of 
unusual drought. Except on very well-known streams pre- 
vious data on this point are generally not available. The 
flow should therefore be measured carefully through the usual 
period of low water during at least one season. From the 
minimum flow thus obtained there are various ways of judging 
the minimum flow in a very dry year. Sometimes certain 
riparian marks are known to have been uncovered in some 
particular year, and the relative flow can be computed from the 
difference thus established. Again, the records of a series of 
years may be obtained from a neighboring stream of similar 
character, and the ratio between ordinary and extraordinary 
minima assumed to be the same for both. This assumption 
must be made cautiously, for neighboring streams often are 
fed from sources of very different stability. 



394 ELECTRIC TRANSMISSION OF POWER. 

Failing in these more direct methods, recourse may be 
taken to rainfall observations. For this purpose the rainfall 
in the basin of the stream should be measured during the con- 
tinuance of the observations on flow. By noting the effect of 
known rainfall on the flow of the strean\, one can make a 
fairly close estimate of the flow in a very dry year in which 
the rainfall is known by months, or for an assumed minimum 
rainfall. In a similar way can be ascertained the probable 
high water mark, record of which is often left by debris on 
the banks. 

In a fairly well-known country the conditions of flow can be 
approximated by reference to rainfall alone. The area drained 
by the stream down to the point of utilization can be closely 
estimated. If rainfall observations in this district are avail- 
able, or can be closely estimated from the results at neighbor- 
ing stations, one may proceed as follows: The total water 
falling into the basin is 2,323,200 cubic feet per square mile 
for each inch of rainfall. Only a portion of this finds its way 
into the streams, most of it being taken up by seepage, evapo- 
ration, and so forth. The proportion reaching the streams 
varies greatly, but is usually from .3 to .6 of the whole. If this 
proportion is known from observations on closely similar 
basins and streams the total yearly flow can be approximated, 
and if the distribution of flow on a similar stream is known, 
one can make a tolerable estimate of the amount and condi- 
tions of flow in the stream under investigation. 

This process is far from exact, since the proportion of the 
total water which is found in the streams varies greatly from 
place to place, and with the total rainfall in any given week or 
day. The sources of loss do not increase with the total pre- 
cipitation, and the only really safe guide is regular observa- 
tion of the rainfall and the flow during the same period. At 
times, however, rainfall estimates are about the only source 
of information available and when made with judgment are 
decidedly valuable. In a well investigated country they are 
sometimes surprisingly accurate. 

A good idea of the uncertainties of hydraulic power can be 
gathered from the recorded facts as regards the Merrimac, 
one of the most completely and carefully utilized American 



HYDRAULIC DEVELOPMENT. 395 

streams, which has been under close observation for half a 
century. The area of its watershed above Lowell, Mass., is 
4,093 square miles and the mean annual rainfall of the region 
is about 42 inches. The observations of many years indicate 
that the maximum, minimum, and mean flows are on approxi- 
mately the following basis: 

(Spring) Maximum, 90 cubic feet per minute per square mile 

(June) Mean, 55 cubic feet per minute per square mile 

(August, September) . .Minimum, 30 cubic feet per minute per square mile 

The annual rainfall, if it all could be reckoned as in the 
stream and uniformly distributed, would amount to very 
nearly 180 cubic feet per minute per square mile of watershed. 
In fact, this flow is reached or passed only on occasional 
days of heavy freshets during the spring rains, when the snow 
is melting rapidly. The normal maximum flow is just 50 per 
cent of the conventional average, while the real average falls 
to about 30 per cent and the minimum to less than 17 per 
cent. Of late years this minimum has sometimes been still 
smaller, little over 10 per cent instead of 17, a state of things 
due to the destruction of the forests on the upper watershed. 
In a heavily wooded country the rainfall is long retained and 
finds its way to the streams slowly and gradually. When 
the forests are cut off the water runs quickly to the streams, 
and the result is heavy seasons of freshets when the snow is 
melting — all the more rapidly because of lack of forest shade 
— and extreme low water during the dry months. In a bare 
country the variations of flow are often prodigious, and with- 
out storage one can safely reckon only upon the minimum 
flow of the dryest year. As the denudation of the uplands goes 
on hydraulic development will steadily grow more expensive. 

One cubic foot per second per square mile of drainage 
area is a figure often used to determine the average flow for 
which development should be planned and in streams like 
those of New England this estimate is not far from the truth. 

In some streams, generally in hot climates, no small part 
of the flow is during the dry season in the strata underlying 
the apparent bed of the stream, and can be in part, at least, 
captured by carrying down the foundations of the permanent 
works. 



396 ELECTRIC TRANSMISSION OF POWER. 

When the flow has been ascertained the available HP is 
easily computed. The practicable head can be easily deter- 
mined by a little leveling. If H is this head in feet and Q 
the flow in cubic feet per minute, then the theoretical HP of 
the stream is 

62.4 H Q 



HP 



33,000 



The mechanical HP obtained by utilizing the stream in water- 
wheels is this total amount multiplied by the efficiency of 
the wheels, usually between .75 and .85. At 80 per cent 
efficiency the proceeding formula reduces to 

HP ^^ 

which gives the available mechanical horse-power directly. In 
many streams the available head is limited by the permissible 
overflow of the banks as determined by the rights of other 
o\vners, or by danger of backing up the stream to the detri- 
ment of powers higher up. These conditions must be deter- 
mined by a careful survey. 

Before taking up seriously the development of a water-power 
it is advisable to enter into an examination of the legal status 
of the matter, which is sometimes very involved. The gen- 
eral principle of property in streams is that the water belongs 
in common to the riparian owners, and cannot be employed by 
one to the detriment of another. But each State has a set of 
statutes of its own governing the use of water for power and 
other purposes, often of a very complicated character, involved 
with special charters to storage and irrigation companies and 
other ancient rights, so that the real rights of the purchaser of 
a water privilege are often limited in curious and troublesome 
ways, especially when the stream has been long utilized else- 
where. 

Generally the riparian owners have full rights to the nat- 
ural flow of the stream, which is often by no means easy to 
determine. The laws of various States regarding the matter 
of flowage vary widely, and altogether the intending purchaser 
will find it desirable to investigate carefully not only the title 



HYDRAULIC DEVELOPMENT. 397 

to the property, but the Hmitations of the rights which he 
would acquire. 

In streams of small volume carried through pipe lines the 
effective head is diminished by friction in the pipes. This 
loss has already been discussed in Chapter II. In any case 
where water is carried in canals or open flumes there will be, 
too, a slight loss of head, generally trivial. 

It often happens that there is so great a difference between 
the normal flow of the stream during most of the year and its 
minimum flow during a few weeks as to make it highly desira- 
ble to store water by impounding it, so as to help out the 
sometimes scanty natural supply. With mountain streams 
under high head this is frequently quite easy. Even when it 
is impracticable to impound enough to help out during the 
whole low water period, it is sometimes very useful to impound 
enough to last for a day or two in case of necessary repairs. 

A certain reservoir capacity is quite necessary, so as to per- 
mit the storage of water at times of light load for utilization 
at times of heavy load. This process is carried out on a vast 
scale on the New England rivers, where the water, used during 
the day in textile manufacturing, is stored in the ponds at 
night as far as possible. While electric transmission plants do 
not offer the same facilities for storage, since they generally 
run day and night, the application of the same process would 
often greatly increase their working capacity and greatly lower 
the fixed charges per hydraulic HP. Such storage is espe- 
cially valuable in cases where the water supply is limited, as it 
often is in plants working under high heads. Every cubic foot 
of water is then valuable and should be saved whenever pos- 
sible. Regulation by deflecting nozzles, which is very generally 
employed in this class of plants, is particularly objection- 
able on the score of economy, and ought to be replaced by 
some more efficient method. 

As an example of what can be done with storage under high 
heads, it happens that at 650 feet effective head oae mechani- 
cal HP requires almost exactly one cubic foot of water per 
minute at 80 per cent wheel efficiency. For a 500 HP plant, 
then, the water required is 30,000 cubic feet per hour. 

One can store 43,560 cubic feet per acre per foot of depth, 



398 ELECTRIC TRANSMISSION OF POWER. 

SO that a single acre 10 feet deep would store water enough to 
operate the plant at full load for 14^ hours, or under ordinary 
conditions of load for a full day. If the flow in the stream 
were only 15,000 cubic feet per hour in time of drought, the 
acre would yield two days supply and 15 acres would carry the 
plant for a month. Such storage is common enough in irriga- 
tion work, and is capable of enormously increasing the work- 
ing capacity of a transmission plant, even at a head much less 
than that mentioned. 

With even 100 feet available head, it is comparatively easy to 
impound water enough to assist very materially in tiding over 
times of heavy load and in increasing the available capacity. 




Fig. 222. 

A survey with storage capacity in view should be made when- 
ever storage is possible, and the approximate cost of storage 
determined. A little calculation will show in how far it can 
be made to pay. 

In general the utilization of a water-power consists in lead- 
ing the whole or a part of a stream into an artificial channel, 
conducting it in this channel to a convenient point of utiliza- 
tion, and then dropping it back through the water-wheels 
into the channel again, usually via a tail-race of greater or less 
length. 

Except where there is a very rapid natural fall a sub- 
stantial dam is necessary, which backs up the water into a 
pond, usually gaining thus a certain amount of head, whence 
the water is led iii an open canal to some favorable spot from 
which it can be dropped back into the channel at a lower level. 



HYDRAULIC DEVELOPMENT. 



399 



The canal may vary in length from a few rods to several miles, 
according to the topography of the country. The tail-race lead- 
ing the water from the wheels back to the stream is short, except 
in rare instances like the great Niagara plant. In this case, 
shown somewhat roughly in Fig. 222, the usual construction 
was reversed. To obtain ample clear space for manufacturing 
sites and the like, the water was utilized by constructing above 
the cataract an artificial fall at the bottom of which the wheels 
were placed. From the bottom of this huge shaft, cut 178 feet 
deep into the solid rock, the water is taken back into the 




Fig. 223. 



river through a tunnel 7,000 feet long, which constitutes the 
tail-race. 

In the case of mountain streams having a very rapid fall, 
the dam is often quite insignificant, serving merely to back up 
the water into a pool from which it may be conveniently drawn, 
and in which the water may be freed of any sand that it carries, 
or even to deflect a portion of the water for the same purpose. 
In such cases the water is usually carried in an iron or steel 
pipe, following any convenient grade to the bottom of the fall 
chosen, at which point its full pressure becomes available. 

In ordinary practice at moderate heads the volume of water 
has to be so considerable for any large power as to make a 
long canal very expensive. Further, it usually happens that 
the topography of the country is such as to make it very diffi- 
cult to gain much head by extending the canal. Thus the 
points chosen for power development must be those where 
there is a rather rapid descent for a short distance — falls or 
considerable rapids. Then a dam of moderate height gives a 



400 ELECTRIC TRANSMISSION OF POWER. 

fair head by simply carrying the canal to a point where the 
water can be readily returned to the stream below the natural 
fall. The more considerable this fall the less need for an 
elaborate dam, which may become simply a means of regulat- 
ing the flow of water without noticeably raising the head. 

A fine example of this sort of practice is shown in Fig. 223, 
which shows a plan of the hydraulic development of the falls 
of the Willamette River at Oregon City, Ore. The river at 
this point gives an estimated available HP of 50,000 under 40 
feet head. The stream plunges downward over a precipitous 
slope of rough basalt, and the low dam which follows the some- 
what irregular shape of the natural fall, is hardly more than an 
artificial crest to guide the water toward the canal on the west 




Fig. 224. 

bank of the stream. This canal has recently been widened, 
and both constructions are shown in the figure. The fine three- 
phase transmission plant of the Portland General Electric Com- 
pany now faces on the new canal wall near the section G. At 
the end of the canal downstream a series of locks lead down to 
the lower river, making the falls passable for river craft. Only 
a small part of the available power is as yet used. 

Almost every river presents peculiarities of its own to the 
hydraulic engineer. Generally the dam is a far more promi- 
nent part of the work than at Oregon City, and adds very 
materially to the head. Choosing a proper site for the dam, 
and erecting a suitable structure, requires the best skill of the 
hydraulic engineer. Bearing in mind that the function of a 
dam is to merely retain and back up the flowing water, it is 
evident that it may be composed of a vast variety of materials 



HYDRAULIC DEVELOPMENT. 



401 



put together in all sorts of ways. Stone, logs, steel, all come 
into play combined with each other and with earth. 

The character of the river bed which furnishes the founda- 
tion is a very important factor in determining the material and 
shape of the dam used. When the bed is of rock or of that hard 
packed rubble which is nearly as sohd, a well-built stone dam 
is the best, as it is also the costhest, construction. For such 
work the way is cleared by a coffer dam and the masonry is 



TOP OF BULKHEAD 



TOP OF SHUTTER AND 

WING DAMS 

<ji^TOP OF DAM IN SHUTTER 

OPENING,SHUTTER RAISED 

BY 5 HYDRAULIC RAM8 




SECTION OF DAM 
j THRUST — IQTI TONS. J 
j STABILITY— 7079 TONS, f 

Containing 37,000 cu. yards of masonry. 
Fig. 225. 

laid, if possible, directly upon the bedrock. When the bottom 
is hard pan a deep foundation for the masonry is almost as 
good as the ledge itself, while on a gravel bottom sheet piling 
is sometimes driven and the stone work built aroimd it. The 
ground plan is very frequently convex upstream, giving the 
effect of an arch in resisting the pressure of the water. Fig. 
224 shows a section of a typical masonry dam, built over sheet 
piling in heavy gravel. This particular dam is 22 feet 6 inches 
high and nearly 300 yards long. The coping is of solid granite 
slabs a foot thick. Below the dam lies the usual apron of 



402 ELECTRIC TRANSMISSION OF POWER. 

timber and concrete, with timber sills anchored into the dam 
itself. The flooring of the apron, of 12 x 12 inch timbers laid 
side by side, is bolted to the foundation timbers laid in the 
concrete. The purpose of this apron, as of such structures in 
general, is to prevent undermining of the dam by the eddies 
below the fall. 

A still finer example of the masonry dam is shown in Fig. 
225 — the great dam of the Folsom Water Power Company 
across the American River at Folsom, Cal. It is built of 
hewn granite quarried on the spot, and is founded on the same 
ledge from which the material was taken. The abutments 
likewise are built into the same ledge. On the crest of the 
dam proper is a huge shutter or flash board, 185 feet long, 
capable of being swung upward into place by hydraulic power. 
When thus raised it gives an added storage capacity of over 
13,000,000 cubic yards of water in the basin above. This 
dam furnishes power for the Folsom-Sacramento transmission, 
now part of the immense network of the California Gas and 
Electric Co., and it ranks as one of the finest examples of 
hydraulic engineering in existence. Including the abutments 
it is 470 feet long, and the crest of the abutments towers 
nearly 100 feet above the foundation stones. Its magnificent 
solidity is not extravagance, for the American River carries 
during the rainy season an enormous volume of water, filling 
the channel far over the crest of the dam when at its maximum 
flow. There are few streams where greater strains would be 
met. 

While these masonry dams are splendidly strong and endur- 
ing, they are also very expensive, and hence unless actually 
demanded for some great permanent work are less used than 
cheaper forms of construction. In many situations these are 
not only cheaper in first cost, but even including deprecia- 
tion. There are divers forms of timber dam which have given 
good service for many years at comparatively small expense. 
Of such dams, timber cribs ballasted with stone are probably 
under average conditions the best substitute for solid masonry. 
These crib dams when well built of good materials, are very 
durable and need few and infrequent repairs. Some such 
dams, replaced after twenty-five or thirty years in the course of 



HYDRAULIC DEVELOPMENT. 



403 




changing the general hydraulic conditions, have shown timbers 
as solid as the day they were put down, and capable of many 
years' further service. 



404 



ELECTRIC TRANSMISSION OF POWER. 



A fine example of such construction is the dam of the Con- 
cord (N. H.) Land & Water Power Company, at Sewall's 
Falls on the Merrimac. A section of this structure is shown 
in Fig. 226. The foundation is in the main gravel, in which the 
dam is made secure by sheet piling and stone ballast. The 
structure is essentially a very solid timber crib with a very long 
apron. The total head is 23 feet, of which more than half is 
due to the dam, as shown in the levels. The apron is armored 
with five-sixteenths inch steel plate, the better to withstand the 
bombardment of stray logs to which it is sometimes subjected. 
The abutments are of granite. It has proved very serviceable, 
having successfully withstood several tremendous freshets with 
no damage save some undermining of one of the abutments, 



2- ^ Rods 

High Water Level 

^'■'i^'?^**^'^' J' I ^ ^^^^ above 

,x.<^2gS^^^-^Thacher Steel Eoda I Crest of Dam 




Fig. 227. 



which has been repaired with crib work. Considering the 
character of the river bed, this dam is probably as reliable as 
one of masonry, and its cost was little over half that of a 
masonry dam. 

For small streams these ballasted timber dams are admir- 
able, and little more is needed in most cases. 

Another very convenient and useful form of dam, of which 
many examples have of late been erected, is shown in Fig. 227. 
It is a concrete and steel dam of the gravity type in which the 
dam is given stability far above that due to its structural 
weight, by the weight of the superincumbent water. It is 
unusual in that it really follows modern architectural lines 
instead of conventional hydraulic construction, and the form 
here shown, devised by Mr. Ambursen, of Watertown, N. Y., 
involves a good many novel features. Fig. 228 gives a section 



HYDRAULIC DEVELOPMENT, 405 

of the dam which is essentially an inclined concrete and steel 
floor supported by concrete buttresses. The dam here shown 
is about 12 feet high and the supporting buttresses are of the 
section indicated in the figure, 12 inches thick and spaced 6 
feet between centres. Along the upstream slope of these 
buttresses there are set in the concrete, connecting the buttresses, 
a series of | inch twisted steel rods about 8 inches between 
centres, and just below them covering the whole slope are 
sheets of heavy ''expanded metal." Over and about this steel 
substructure is laid a tight concrete floor about 6 inches thick, 
merging at the toe of the dam into a massive concrete shoe 
filling the space between the buttresses and built upon the 
foundation ledge. Near the top of the dam the slope is made 
with a hard finish of rich concrete and the top itself is made 
extra heavy to resist the rush of the water. 

These dams are sometimes built with concrete downstream 
faces and aprons, and in fact may take any form that occasion 
requires. They are tight and strong, and ought to prove 
durable, while the cost is usually little more than that of a 
timber crib. They are, like concrete work generally, quickly 
erected, and seem specially adapted to long runs of moderate 
height, although they are being used for heights of 30 feet 
and more, and when properly designed would appear to be as 
generally applicable as any other construction. A similar 
construction can sometimes be advantageously used for fiumes 
and canal walls, since concrete work can be done with material 
easily available, and with a very small proportion of skilled 
labor, and when well done is both strong and durable. 

The type of dam selected for any particular case is governed 
by the hydraulic requirements and the conditions at the 
proposed site, and the relative costs can only be settled by 
close estimates. Sometimes massive rubble masonry is about 
as cheap as anything else, while in other circumstances con- 
crete or timber would show the minimum cost. 

The canals leading the water to the wheels are of construc- 
tion as varied as the dams, depending largely on the nature of 
the ground. Sometimes they are merel}^ earthwork, oftener 
they are lined with timber, concrete, or masonry. Canal con- 
struction is a matter to be decided on its merits by the 



406 



ELECTRIC TRANSMISSION OF POWER. 



hydraulic engineer, and very little general advice can be 
given. For low heads wooden pipes made of staves like a bar- 
rel and hooped with iron every 3 or 4 feet are sometimes used. 
In many situations this construction is cheaper than steel pipe 
and answers admirably. Such wooden pipes are considerably 
employed in the West, the material being generally redwood, 
and have proved remarkably durable, some having been in use 




Fig. 228. 

for more than twenty years. Open timber flumes are also 
widely used. 

For very high heads, canals and flumes are almost univer- 
sally replaced by iron or steel riveted pipe taken by the nearest 
route to the wheels below. This practice has been general 
on the Pacific coast and has given admirable results. The 
pipes are asphalted inside and out to prevent corrosion, and 
some pipe lines have been in service for a quarter of a cen- 
tury without marked deterioration. Large pipes and those for 
very heavy pressures are usually made of mild steel. The 







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1000 ft. 

Fig. 229. 



pipes are customarily made in sections for shipment, from 
20 to 30 feet long, and the slip joints are riveted or packed on 
the ground. For transportation over very rough country and 
for very large pipes, the sections may be no more than 2 or 
3 feet long. The joints are then asphalted on the ground. 
Fig. 228 shows several of these short sections joined together, 
exhibiting the nature of the riveting and the terminal slip 
joint. 



HYDRAULIC DEVELOPMENT. 



407 



In running such a pipe line it is usually taken in as straight 
a course as possible, and is laid over, on, or under the ground as 
occasion requires, usually on the surface, conforming to its gen- 
eral contour. In long lines the upper end is somewhat larger 
and thinner than the lower, which has to withstand the heavy 
pressure. Fig. 229, which is a profile to scale of the pipe line 
of the noted San Antonio Canon plant in southern California, 
gives an excellent idea of good modern practice in this sort of 
work. There is here a total fall of about 400 feet in a distance 
of 2,000 feet. The main pipe is 30 inches in diameter, and the 
steel is of the gauges indicated on the various sections. At 




fibowiag-method x>t anchoring pipe on » steep 
SitdeviUi examples ol lead and slip joints . 



Fig. 230. 



the crests of two undulations, air valves are placed to ensure 
a solid and continuous column of water in the pipe. The last 
540 feet of pipe is reduced to 24 inches and the gauge of steel 
is somewhat heavier. The total length of the pipe line is 2,370 
feet. To protect the pipe against great changes of tempera- 
ture it was loosely covered with earth, rock, and brush when- 
ever possible. At two sharp declivities the pipe was anchored 
to the rock. 

The general method of anchoring on a steep incline is shown 
in Fig. 230. In this case the slip joint is simply calked, and 
where consecutive sections are at an angle, a short sleeve is 
fitted over the joint and lead is run in as shown in the cut. 



408 



ELECTRIC TRANSMISSION OF POWER. 



Often a packed slip joint is used very freely, thereby gaining 
in flexibility, and riveted joints may be only used occasionally. 
The line is generally started from the lower end and the joints 
or the whole interiors of the sections asphalted as they are laid. 
The following table gives the properties of steel hydraulic 
pipe of the sizes in common use, and double riveted: 



Diameter in inches 


10 


13 


14 


16 


18 


20 


24 


30 


36 


43 


Area in square inches. 


78 


113 


153 


201 


254 


314 


452 


706 


1,017 


1,385 


Cubic feet per minute at 
three foot per second 


100 


142 


200 


255 


320 


400 


570 


890 


1,300 


1,760 


Weight in pounds per 
foot 


19.25 


22.75 


26 


29.5 


34 


36.5 


43.5 


54 


67 


74 5 


Safe head in feet 


900 


750 


650 


560 


500 


450 


375 


300 


150 


135 


Change in safe head for 
each gauge number. . 


100 


90 


80 


70 


60 


55 


45 


35 


20 


20 



The pipe is assumed to be of No. 10 gauge steel, and the 
changes in safe head are of course approximate only, but hold 
with sufficient exactness for a variation of four or five gauge 
numbers. It is better to use a pipe too thick than one too 
thin, and to use extra heavy pipe at bends. Where the ground 
permits, the water can often be carried to advantage in a flume 
or ditch, and then dropped through a comparatively short pipe 
line. For heads approaching or surpassing 1,000 feet it is prob- 
ably safer to use lap-welded tube for the lower portion of the 
run. In every case suspended sand must be kept out of the 
water, else it will cut the wheels and nozzles like a sand blast. 
When one remembers that under 400 feet head the spouting 
velocity of the water is about 160 feet per second, the need of 
this precaution is evident. A large settling tank is usually 
provided at the head works, spacious and deep enough to let 
the pipe draw from the clear surface water. At its lower end 
the pipe line terminates in a receiver — a heavy cylindrical 
steel tank of considerably larger diameter than the pipe proper, 
from which water is distributed to the wheels. 

On very high heads a relief valve is attached at or near the 
receiver to avert danger from a sudden increase in pressure in 
the pipe, such as might be caused by some sudden obstruction 
at the gate. 



HYDRAULIC DEVELOPMENT. 409 

This pipe line method of supply is considerably used for 
turbines of moderate size on heads as low as 75 to 100 feet, in 
cases where the natural fall of the stream is rather sudden. It 
really amounts to a considerable elongation of the iron pen- 
stock which is in common use. Whenever there is a sharp 
declivity in difficult country, pipii^g is often easier and cheaper 
than constructing a sinuous flume or canal. In such situa- 
tions the pipes may be 5 or 6 feet in diameter or even more, 
and being under very moderate pressure, may be comparatively 
light and cheap. 

In cold climates ice is one of the difficulties most to be 
dreaded in hydraulic work. In high-pressure pipe lines there 
is little to fear, for fast-running water does not freeze easily 
and the pipes can generally be readily covered, as in the San 
Antonio Canon plant, enough to prevent freezing. Large 
canals simply freeze over and the interior water is thus pro- 
tected. But in cold climates there is considerable danger of 
the so-called anchor-ice. This is, in extremely cold weather, 
formed on the bed and banks of rapid and shallow streams. 
The surface does not freeze, but the water is continually on 
the point of freezing and flows surcharged with fine fragments 
of ice that pack and freeze into a solid mass with the freezing 
water rapidly solidifying about it. When in this condition it 
rapidly clogs the racks that protect the penstocks, and even 
the wheel passages themselves. In extremely cold climates 
under similar circumstances the water becomes charged with 
spicular ice crystals known as frazil in Canada, far worse to 
contend with than ordinary anchor-ice. 

The best protection against ice is a deep, quiet pond 
above the dam, in which no anchor-ice can form, and which 
will attach to its own icy covering any fragments that drift 
down from above. In case of trouble from anchor-ice, about 
the only thing to do is to keep men working at the racks with 
long rakes, preserving a clear passage for the water. If the 
wheel passages begin to clog there is no effective remedy. 

The most dangerous foe of hydraulic work is flood. The 
precautions that can be taken are, first, to have the dam and 
head-works very solid, and second, so to locate them if possible 
as to have an adequate spillway over which even a very large 



410 ELECTRIC TRANSMISSION OF POWER. 

amount of surplus water can flow without endangering the 
main works. If a pipe line is used it must be laid above high 
water mark, else the first freshet will probably carry it away. 
The power station must likewise be out of reach even of the 
highest water. 

Closely connected with the subject of floods is that of varia- 
ble head, which in many streams is a constant source of diffi- 
culty. In times of flood the extra height of the water above 
the dam is generally useless, while the tail-water rises and 
backs up into the wheels, cutting down their power and speed, 
often very seriously. This matter has already been discussed 
in Chapter IX, in so far as it is connected with the arrange- 
ment of the turbines. At very high heads this trouble van- 
ishes, as no possible variation of the water level can be a 
considerable fraction of the total head. 

The most delicate questions involved in hydraulic develop- 
ment are those connected with variable water supply. Having 
ascertained as nearly as possible the minimum flow, the mini- 
mum natural continuous supply of power is fixed, but it remains 
to be determined how the water in excess of this shall be 
utilized, if at all. 

Three courses are open for increasing the available mer- 
chantable power. First, water can be stored to tide over the 
times of small natural supply. Second, a plant can be installed 
to utilize what water is available for most of the year and can 
be curtailed in its operation during the season of low water. 
Third, the service can be made continuous by an auxiliary 
steam plant in the power station. Storage of water can 
obviously be used in connection with either of the other 
methods. 

Under very high heads storage is always worth undertaking 
if the lay of the land is favorable. This of course means a 
dam, but not necessarily a very high or costly one. If possible 
the storage reservoir should be a little off the main flow of the 
stream so as to escape damage from freshets. Reverting to 
our previous example of storage, suppose we have 500 HP 
available easily for nine months of the year, but a strong 
probability of not over 250 HP for the remaining three months. 
We have already seen that under these circumstances . 15 



HYDRAULIC DEVELOPMENT. 411 

acres flooded 10 feet deep will keep up the full supply for a 
month. If say 50 acres can be thus flooded, the all-the-year- 
round capacity of the plant will be doubled. In the moun- 
tainous localities where such heads are to be found, land has 
usually only a nominal value, and impounding the equivalent 
of this amoimt of water is frequently practicable. If it can be 
done at say a cost of $75,000, the annual charge per HP stored, 
counting interest and sinking fund at 8 per cent, will be $24, 
and the investment would generally be a profitable one. If 
the storage cost $100,000, the annual charge would be $32, 
and this would not infrequently be well worth the while, when 
power could be sold for a good price. 

At lower heads the annual charge per HP stored would be 
considerably greater for the same total expenditure. Some- 
times, however, storage capacity can be much more cheaply 
gained for both high and low heads, at for instance not more 
than half the charge just mentioned. The matter is always 
worth investigating thoroughly when there is doubt about 
supplying the power market with the natural flow. The points 
to be looked into are the nature and extent of the low water 
period, and the cost of developing various amounts of storage 
capacity. Sometimes the period of extreme low water is 
much shorter than that assumed, and storage is correspond- 
ingly cheaper. 

There are some cases in which it is possible to supply cus- 
tomers with power for nine or ten months in the year, falling 
back on the individual steam plants in the interim. When 
transmitted power can be cheaply had, it is worth while for 
the power user who is paying say $100 per HP per year for 
steam power, to take electric power at $50 per HP per year for 
nine months, and to use steam the other three months. Certain 
industries, too, are likely to be comparatively inactive in mid- 
summer, or may find it worth while to force their output 
during the months when cheap power is obtainable, and shut 
down or run at reduced capacity when the power is unavail- 
able. This is a matter very dependent on local conditions, 
and while the demand for such partial power supply is gener- 
ally limited, there are many cases in which it would be advan- 
tageous for all parties concerned. In some mountainous 



412 ELECTRIC TRANSMISSION OF POWER. 

regions, winter is the season of low water owing to freezing, 
and various industries are suspended which may be profitably 
supplied with power when the winter unlocks its gates. 

Eking out the water supply by an auxiliary steam power 
station is likewise not of general applicability, but sometimes 
may prove advantageous. It is most Hkely to prove useful in 
localities where a steam power plant would pay by virtue of 
the economy due to production on a large scale and distribu- 
tion to small users. Cheap water-power a large part of the 
year then abundantly justifies adjunct steam-power when 
necessary. The moral effect of continuous power supply 
is valuable in securing a market. Whether such a supply is 
profitable depends on the ratio between the cost of water- 
power and the cost of steam-power. And it must not be for- 
gotten that steam-pov^er for two or three months in the year is 
relatively much more costly than continuous power. 

The general charges are the same, although labor, coal, and 
miscellaneous supphes decrease nearly as the period of opera- 
tion. Consequently, since there is this large fixed item, 
amounting to from 20 to 40 per cent of the total annual cost, 
the cost of power in a plant operated only three months will be 
relatively at least 50 per cent greater than if it were in con- 
stant operation. There must be a large margin in favor of 
water-power to justify this auxiliary use of steam, unless the 
latter would pay on its own account, as for instance in a plant 
used largely for lighting, which would be the most profitable 
kind of electric service were there a sufficiently large market. 
A large lighting load increases the peak considerably, but, as 
compensation, drops the peak notably during summer when 
low water generally comes. Particularly is this the case with 
public lighting which in summer does not overlap the motor 
load. 

The fundamental questions to be asked in taking up the 
supplementary steam plant are first, how large a plant is it 
advisable to install, and second, how much energy will it have 
to contribute to the common stock. To determine the answers, 
the distribution of flow must be pretty closely known. The 
hydraulic power may fail either by drought or by flood. If 
the former, there is likely to be a period of a couple of months 



HYDRAULIC DEVELOPMENT. 



413 



in which the power will be subnormal, perhaps half to two- 
thirds of the average supply. To carry a full load over this 
period, implies a steam plant of say half the full capacity of the 
hydraulic plant, in operation during a portion of the time for 
two months. To put things on a concrete basis, suppose a 
2,000 HP hydraulic plant, with a 1,000 HP supplementary 
steam plant. As stations ordinarily run, the load for a con- 
siderable part of the day is much less than the maximum load. 
Fig. 231 shows the actual load curve for three successive days 
on a high voltage transmission plant doing a mixed power and 



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Fig. 231. 



2 A.M. 



6 A.M. 



lighting business. It shows a load factor of very nearly 50 per 
cent, and if the peak of the load corresponded to the full capacity 
of the hydraulic plant, during the period of low water under 
our assumed conditions, the steam plant would have to fur- 
nish, not one-half the full capacity of the plant, but merely 
the energy above the half load ordinate of the load diagram. 
Considering this portion of Fig. 231, it will be seen that while 
at the peak of the load the steam plant would be working at 
full capacity, it would have to be in use only about half the 
time at a load factor again of about 50 per cent. So it appears 
that for about two months the 1,000 HP steam plant will be 
called upon for only about one-fourth of the actual energy 



414 ELECTRIC TRANSMISSION OF POWER. 

delivered by the plant, say between four and five per cent of 
the year's output. 

Therefore, the economic question is the cost of furnishing 
5 per cent of the yearly output of a steam plant of half the 
total hydraulic capacity worked at about half load. This is 
a very different case from any ordinary estimate on the cost 
of steam-power. 

Taking the cost of the supplementary steam and electric 
plant in this case at $60,000 and counting interest, depreciation, 
and other fixed charges at 10 per cent, there is an annual 
charge of $6,000 against the plant even if it be not run at all. 
At the assumed load conditions it will cost at least $25 per day 
for fuel, supplies, and extra labor during the two months of 
low water so that the upshot of the matter is, that it will cost 
not less than $7,500 yearly to raise the limit of capacity of 
the plant from 1,000 HP to 2,000 KP. 

But since the plant load factor is 50 per cent, the average 
output is raised only from 500 to 1,000 HP. Even on this 
basis the supplementary plant evidently will pay at the ordi- 
nary prices of fuel and of electric energy, but it is equally 
clear that as the required supplementary plant grows rela- 
tively larger, and the proportion of steam generated output 
increases, a point will soon be reached at which the added 
annual cost cannot be compensated by increased possible 
sales of power; provided of course, that the price of power 
sold is below that at which it could profitably be generated 
by steam alone on a similar scale. 

To look at the matter from another side, in the case we 
have been considering, the total effect of supplying part of 
the output by steam would probably be to increase the cost of 
the year's output by less than 15 per cent, and the supplemen- 
tary plant would pay handsomely. Probably in most cases 
it would still be profitable even if 10 per cent of the total out- 
put were due to steam. As this proportion increases, the 
advantage diminishes, and finally a point is reached at which 
any further use of steam cuts down profits rapidly. Each 
case must be worked out by itself, as the result depends upon 
local conditions, and generalizations are therefore unsafe. 
It is usually the fact however that a stream can be developed 



HYDRAULIC DEVELOPMENT. 415 

for its flow in early summer with entire safety, leaving the 
minimum flow to be cared for by a supplementary plant. 

In a few instances it is practicable to connect the power 
station generators so as to be operated either by steam or by 
water-power, thus saving the cost of extra generators. But 
it is generally better to place the supplementary plant in a 
substation at the receiving end of the line, thus allowing it 
to serve as an auxiliary plant in case of accident to the line or 
needed repairs to the hydraulic works. Turbo generators from 
their economy of space and their readiness for operation are 
well adapted for use in supplementary plants, and for such 
use it does not pay to install a costly boiler plant. 

Steam-power on a 12 hour basis at steady full load varies 
according to the size and kind of plant, cost of fuel, and so 
forth, from a little under $20 per HP year to $125 or more, 
with an increase of one-third to one-half in case of variable 
loads. Water-power fully developed rents for from $5 to $50 
or more per HP year, and may cost to develop anywhere from 
$20 to $150 per HP. At the former price it is cheaper than 
steam under any circumstances ; at the latter it is dearer than 
steam unless the fuel cost is abnormally high. 

If the cost of hydraulic development can be kept below 
$100 per HP, water-power can nearly always drive steam-power 
out of business. 

With respect to the prime movers to be employed in a 
hydraulic development, one must be governed largely by cir- 
cumstances. The choice in general lies between turbines and 
impulse wheels, the properties of which have been fully dis- 
cussed in Chapter IX. Without attempting to draw any hard 
and fast lines, turbines are preferable up to about 100 feet head, 
unless very low rotative speed is desirable, or very little power 
is to be developed. Above that, the impulse wheels grow 
more and more desirable, and above 200 feet head the field is 
practically their own. It is generally practicable and desirable 
to use wheels with a horizontal axis. Only in a few instances is 
it necessary to resort to a vertical axis, as when there is consid- 
erable danger of the tail-water rising clear up to the wheels, or 
when, as at Niagara, a very deep wheel pit is employed. 

The line of operations in developing a water-power subse- 



416 ELECTRIC TRANSMISSION OF POWER. 

quent to the reconnoissance has already been indicated. After 
the more general considerations have been determined, comes 
the question of utilization. 

It may seem needless to suggest that the first thing neces- 
sary is an actually available market, but the author has more 
than once had imparted to him, under solemn pledge of secrecy, 
the location of '' magnificent'' water-powers which could be 
developed for a mere song, located a hundred miles from 
nowhere — out of effective range even of electrical transmission. 

Having a possible market, the next thing is to investigate 
it thoroughly. The actual amount of steam-power must be 
found, together with its approximate cost in large and in 
small units. This information ought to be extended to at 
least an approximate list of every engine used and the nature 
of its use, whether for constant or variable load, whether in 
use throughout the year or only at certain seasons. These 
more minute data are not immediately necessary, but are 
immensely useful later. If it is proposed to include electric 
lighting in the scheme, an estimate of the probable demand 
for lights should be carefully made. A fair guess at this can 
be made from the number of inhabitants in the city or town 
supplied. Where there is competition only with gas, experi- 
ence shows that the total number of incandescent lights installed 
is likely to be, roughly, from one-fourth to one-sixth of the 
population, occasionally as many as one-third, or as few as 
one-eighth. In cities of moderate size it is usually found that 
even with competition from gas, the annual sales of electricity 
for all purposes can with proper exploitation be brought up to 
from $1.50 to $2.00 per capita. This amount may be increased 
by 50 per cent under favorable conditions. 

From the data thus obtained one can estimate the general 
size of the market, and hence the approximate possible demand 
for electrical energy. With this in mind, further plans for the 
hydraulic development can be made. It may be that the 
water-power is obviously too small to fill the market, if so, it 
should be developed completely. If not, much judgment is 
necessary in determining the desirable extent of the develop- 
ment. Probable growth must be taken into account, but it 
cannot safely be counted upon. If steam-power is very 



HYDRAULIC DEVELOPMENT. 417 

expensive most of the engines can probably be replaced by 
motors. The replacement of one -half of them is, under 
average circumstances, a sufficiently good tentative estimate. 

With this as a basis, approximate estimates of the hydraulic 
development can be made. This should be done by a compe- 
tent hydraulic engineer. If the developemnt is easy it is well 
to make estimates for a liberal surplus power also. At this 
stage it is best to have the hydraulic and the electrical engi- 
neer work hand in hand to estimate on the delivery of the 
assumed amount of power. From these estimates the general 
outlook for returns can be reckoned. 

Before actually beginning work it is advisable to make a 
pretty thorough preliminary canvass of the market, to see 
what can be done immediately in the sale of power and light. 
With the certain and the probable consumption ascertained, 
the hydraulic and electrical engineers can work their plans 
into final shape and prepare final estimates. 

All this preliminary work may at first sight seem rather 
unnecessarily exhaustive, but mistakes on paper are corrected 
more easily than any others, and the investigation is likely to 
save many times its cost in the final result. 

Whatever is done should be done thoroughly. Poor work 
seldom pays anywhere, least of all in a permanent installation, 
and it should be conscientiously avoided. 

Above all, continuity of service has a commercial value that 
cannot be estimated from price lists. If it anywhere pays to 
be extravagant, it is in taking extreme precautions against 
breakdowns and in facilities for quick and easy repairs in case 
of unavoidable accident. This applies alike to the hydraulic 
and the electrical work. If the first severe freshet demoral- 
izes the hydraulic arrangements, or the plant runs short of 
water at the first severe drought, a damage is done that it 
takes long to repair in the public mind. On the other hand, 
careful, thorough work, coupled with intelligent foresight, 
insures that complete reliability that is the mint mark of 
honest and substantial enterprises. 



CHAPTER XI. 

THE ORGANIZATION OF A POA\'ER STATION. 

The first thing to be determined in planning a power station 
is the proper site, which should, if steam be the motive power, 
be settled by convenience with respect to the supply of coal 
and water. In using water-power the position of the station 
should be determined in connection with the hydraulic devel- 
opment. Near the foot of the working fall is the natural site, 
but, particularly in mountainous regions, it may be quite im- 
practicable on account of lack of available space, unsuitable 
ground for foundations, inaccessibility, or more often danger 
of flood. Under high heads where a pipe line is used, one 
has a considerable amount of freedom in determining the site, 
since the pipe can be extended and led around to convenient 
locations at moderate expense, say not more than $3 or $4 per 
foot. A relatively small sacrifice of head, too, may enable one 
to secure an admirable location. 

On low heads there is far less latitude permissible, since the 
canal and tail-race are relatively costly, and a change of level 
is a serious matter. 

The proper location and design of a power house calls for 
great tact and judgment. Often hampered by the topographi- 
cal conditions, the site selected must be such as to secure good 
operative conditions at minimum cost. It is well in approach- 
ing the subject to put aside all preconceived notions as to 
how a plant should look, and to remember that it is a strictly 
utilitarian structure. On the hydraulic side it should have 
easy access and exit of the water with the minimum loss of 
head, the shortest feasible penstocks, and the greatest security 
from variations of head. On the electrical side it must be 
dry and clear of floods, conveniently arranged for all the appa- 
ratus, and with an easy entrance for the transmission lines. 
Withal it must have solid foundations, must often be capable 
of easy future extensions and must meet all these require- 
ments at the minimum expense. 

418 



THE ORGANIZATION OF A POWER STATION. 419 

Some of these conditions tend to be mutually exclusive. 
When a plant is built at once to the full capacity of the hydrau- 
lic privilege, the conditions are considerably simplified, but 
this is not the usual case. The plants incidentally described 
in this chapter have been chosen as illustrative of some of the 
problems of power-house organization rather than as models 
of any recognized canons of design. There are no such, save 
in the very general way already indicated, and few of the 
plants erected are not in some particular open to severe criti- 
cism. 

But their design has been necessarily a compromise, and 
more often than not, the objectionable features have resulted 
from following some fashion set by some conspicuous plant 
working under different conditions. The best watchwords in 
power-house design are, safety, operative simplicity, and 
accessibility. Heeding these, with a keen eye to local peculi- 
arities one is not likely to go far astray. 

If possible, the power station should be placed well off the 
main line of flow, or with the main floor well above high water 
mark. The foundations must be of the best to secure safety 
from floods and a proper support for the moving machinery. 
To meet these conditions is not always easy, particularly 
when the available head is low, and sometimes extreme artiflcial 
precautions have to be taken against flood. Such a case is 
found in the Oregon City plant already mentioned, of which a 
sectional view is given in Fig. 232, showing the foundations, a 
single generator, its wheels, and their appurtenances. The 
inner wall of the station is here the outer wall of the canal, 
and both walls and foundations are built very solidly of 
masonry and concrete. In the cut A and B are the draft 
tubes belonging respectively to the wheel cases D and F, which 
are supplied by the penstocks C and E. F contains the regu- 
lar service turbine, a 42 inches Victor wheel coupled direct to 
the generator at P. On the pedestals G above this wheel is a 
ring thrust bearing at / and an hydraulic thrust bearing K. 
Above this is a pulley Y, 6 feet in diameter, and still above 
this the upper bearing support, the bearings N and 0, the 
coupling M, and pedestals Q. 

The wheel case D contains a 60 inch wheel with bearings, pul- 



420 ELECTRIC TRANSMISSION OF POWER, 




Fig. 232. 



THE ORGANIZATION OF A POWER STATION. 421 

ley, and so forth, R, S, W, T, U. The fimctioii of this wheel and 
its attachments is to supply power at the seasons of very high 
water, sometimes several years apart. When the tail-water 
backs up so far that the smaller wheel is no longer equal to the 
work, the generator shaft is arranged to be uncoupled just 
above the wheel. Then the belt tightener X can be brought 
into use, the large wheel started, and the generator driven by 
the horizontal belt. The belt tightener is operated by hand 
wheels at E^ and D^, while similar hand wheels at C2 and B^ 
enable the wheels to be regulated by hand when desirable. 
The governing is normally accomplished by the automatic 
regulator A^. F^ is one of the main race gates, lifted by the 
mechanism at G2. The wheel room is lighted by water-tight 
lieavy glass bulls eyes at Z, each three feet in diameter. The 
dynamo room is lighted by side windows and monitor roof, and 
is fitted with a twelve ton travelling crane K^, carried on the 
supporting column M^ and N2. The penstocks pass through 
the heavy cement floor of the wheel room, J 2, with water-tight 
joints. The main point of interest in this station for our pres- 
ent purpose is not the very complicated and cumbersome 
hydraulic plant but the structure of the wheel room, which 
forms a massive permanent coffer dam securing the motive 
power against all direct interference by even the fiercest 
floods. Such a construction is somewhat inconvenient, but in 
some instances is almost absolutely necessary. The design of 
this plant is unique, in some respects uniquely bad from the 
standpoint of general practice, but many of its peculiarities are 
the result of its situation and of unusual conditions of water 
supply which forced the use of uncommon remedies. Generally 
such extreme measures need not be taken, although since it is 
usually desirable to have the dynamos on a level with the 
wheels, and coupled to them, a water-tight wall between the 
dynamo room and the wheel room is rather common. Quite 
as often, however, full reliance is placed on the strength and 
tightness of the penstocks and wheel cases, and wheels and 
dynamos are placed in the same room. A plant so arranged 
is cheap and simple, and where there is no unusual danger 
of flood is sufficiently secure. Fig. 233 shows a good typical 
plant of this sort, consisting of three double horizontal tur- 



422 



ELECTRIC TRANSMISSION OF POWER. 



bines under 50 feet head, each directly coupled to its generator. 
Each pair of wheels gives 560 HP at about 430 revolutions per 
minute. This represents construction as straightforward and 
simple as that of Fig. 232 was difficult and intricate. It is 
specially interesting in the arrangement of several wheels to 
discharge into a common tail-race, instead of into several 




Fig. 233. 



costly arched tail-races extending under the dynamo room, a 
construction sometimes quite unnecessarily employed. 

The hydraulic conditions may drive the engineer to all sorts 
of expedients, but the main points are security against being 
drowned out, and good foundations. If the dynamos and 
wheels can be given direct foundations of masonry and con- 
crete, such as the former have in Fig. 233 and the latter in Fig. 
232, so much the better. If moving machinery must be carried 



THE ORGANIZATION OF A POWER STATION. 



423 



on beams, support these beams as in Fig. 233, directly under the 
load, by iron pillars or masonry piers. For direct coupling it 
is preferable to have foundations entirely secure from vibration. 
If such cannot be had one may resort successfully to a flexible 
coupling, very often desirable in driving from water-wheels, 
and sometimes rope or belt driving is advisable. 

The proper site having been selected, the next consideration 
is the form of the structure itself. As a rule, whatever the 




Fig. 234. 



nature of the power units, they are most conveniently put, in 
a water-power plant, side by side in a single row with their 
shafts parallel. This placing enables the hydraulic plant to be 
simply and conveniently arranged, and enables the operator to 
take in the whole plant at a glance and watch all the apparatus 
simultaneously. Fig. 234 shows the original ground plan of 
the great Niagara station, well exemplifying this arrangement. 
In stations employing horizontal turbines such a distribu- 
tion of units has even greater advantage in avoiding long and 



424 



ELECTRIC TRANSMISSION OF POWER. 



crooked penstocks. Fig. 233 forcibly suggests the difficulty of 
setting the generators otherwise than in a single row. 

There are, however, not infrequent cases in which the gen- 
erators can be more conveniently placed otherwise. Some- 
times the site desirable for hydraulic reasons is cramped so 
that a power house cannot readily be lengthened enough to 
place the machines in a single row, and even when there is 
space enough considerations of speed may compel a greater 
number of units than can readily thus be accommodated. 
Fig. 235 which is a floor plan of the great Canadian plant at 




'^/?////A/////^T/f//////^777i77y?y///////y.'//yy77z^////i^'%'77//////y^/////W7f//// 

Fig. 235. 

Shawinigan Falls is a case in point. Here the units are dis- 
posed in echelon, which gives a shorter and wider power house 
than usual, and room for extension lengthwise. It should be 
noted here that the switchboard is in a raised gallery over- 
looking the station as in the Niagara plant, and that the 
raising transformers are in a separate adjacent building, to- 
gether with the lightning arresters. 

In some cases the generators may well be put in two short 
rows facing each other, an arrangement sometimes giving a 
far more compact power house than the single line, which is 
inconveniently long when many machines are installed. 



THE ORGANIZATION OF A POWER STATION. 425 

The main thing is to get the generators so placed as to be 
easily watched when in operation and extremely accessible in 
case of accident or of necessary repairs, while the hydraulic 
arrangements are still as simple as possible. Sometimes the 
power house can be greatly cheapened by avoiding the common 
arrangement which calls for tail-races, usually of arched mas- 
onry extending clear under the building. It is not uncom- 
mon to find the foundation masonry in such cases costing very 
much more than the superstructure, and involving much 
expense really needless. 

In general the building erected for a power station should 
be light, dry, fireproof, and well ventilated. Dynamos usually 
run hot enough, without boxing them up in a close room. 
There should be plenty of space back of the row of dynamos, so 
that if machinery has to be moved there will be ample room. 
On the other hand the row of dynamos should be fairly compact, 
as a needless amount of scattering of the machines makes them 
hard to look after. In very many cases one story in height is 
quite sufficient, and in all cases it is preferable to more, so far 
as working apparatus is concerned. Sometimes a second story 
can be well utilized for store rooms, transformer room, and 
quarters for the operating force, but as a rule a single story 
allows more complete accessibility — one of the most important 
features in station design. As land is seldom dear around a 
station for power transmission ample floor space is easily 
obtained, except in occasional cramped localities. A brick 
structure with iron roof is perhaps the most satisfactory kind 
of station. In some situations rubble masonry or concrete 
and steel constructions are convenient. Avoid wood as far 
as is practicable, at least in every place near dynamos, or 
wiring of any kind. A second story, if used at all, should 
have a fireproof floor. Sometimes from temporary necessity 
a frame building is used, but even this can be made fairly safe 
by keeping the machines and wiring clear of wood. In any 
case the floor is the most troublesome part of a station to fire- 
proof. Probably the best material is hard finished concrete, 
or artificial stone with only so much wood covered space as is 
needed to keep it from being too cold, or slippery, or to pro- 
tect it temporarily in moving about machines. Window space 



426 ELECTRIC TRANSMISSION OF POWER, 

should be large and arranged so as to avoid leaving dark 
corners around the apparatus. There should be, too, ample 
door space to facilitate replacing apparatus — nothing is 
more annoying than to be short of elbow room when moving 
heavy machinery. 

For the same reason a good permanent road should be built 
to the power station if one is not already in existence. In 
mountainous regions this is sometimes impracticable, but 
money spent in improving the road is better invested than 
when put into special sectionalized apparatus. It is quite 
possible so to sectionalize a generator of several hundred 
KW that the parts can all be carried on mule back, but the 
expense is considerably increased, and the great advantage of 
having a standard type of apparatus has to be abandoned. 
Hence, unless the cost of improving the road to admit of trans- 
porting ordinary apparatus is decidedly greater than the differ- 
ence in cost between regular and sectionalized machinery, the 
former procedure is advisable. Of course when it comes to a 
question of long mountain trails, sectionalized machinery some- 
times has to be employed. The armature of a polyphase 
machine for use with transformers can very easily be section- 
alized, but if for high voltage or of very large size it is better 
to send in core plates and other material in bundles and wind 
the armature on the spot. 

Having determined the general location and nature of the 
power station, one may take up further arrangements as 
follows : 

I. Motive Power. 
II. Dynamos. 

III. Transformers. 

IV. Accessories. 

The fundamental question is the proper size and character 
of power units. In direct coupled work, prime mover and 
generator must be considered together. In steamdriven 
stations for power transmission the boiler plant may be 
determined by itself, but dynamos and engines should be taken 
up conjointly. 

There is at present rather too strong a general inclination 
to use direct coupled units at any cost. Direct driving is 



THE ORGANIZATION OF A POWER STATION. 427 

beautifully simple and efficient when conditions are favorable, 
and for large units is necessary, but belt and rope driving gives 
singularly little trouble, and when well engineered wastes very 
little energy — not over 3 to 5 per cent for a single direct 
drive, which can almost invariably be used. It is very easy 
to lose far more than this in using a dynamo designed for a 
speed imsuited for its output, or wheels working under dis- 
advantageous conditions. Cases of such misfit combinations 
are not imcommon, and while the workmanship and results 
are often good the engineering is faulty. A very character- 
istic example is shown in Fig. 236, from the power plant of 
an early single phase transmission for mining purposes. The 
generator selected was a 120 KW Westinghouse machine of 
standard form and excellently adapted for its purpose. Its 
speed was 860 revolutions per minute, and to obtain this from 
a working head of 340 feet a battery of four 21 inch Pelton 
wheels was required. Now the Pelton wheel under favorable 
conditions is unexcelled as a prime mover in convenience and 
efficiency, but these conditions were distinctly unfavorable. 
The same work could have been done by a single wheel four or 
five feet in diameter at not over one-third the initial expense 
for wheels and fittings, and at enough higher efficiency to 
more than compensate for the slight loss of energy in a 
simple belt drive. In this case wheel efficiency was sacrificed 
to the speed of the generator. An error quite as common 
is to sacrifice generator efficiency to the speed of the prime 
mover. 

The most flagrant case of this kind that has come to the 
author's notice, was a polyphase machine of less than a hundred 
KW output direct coupled to a vertical shaft turbine at 20 
revolutions per minute. This was of course a I'ow frequency 
machine, but an instance nearly as bad may be found in the 
case of a 75 KW alternator for 15,000 alternations per minute 
direct coupled to an engine at a little less than 100 revolutions 
per minute. These are extreme examples, of course, such 
machines costing several times more than normal generators 
of the same capacity, and having probably fully 10 per cent 
less efficiency. It is, however, not rare to find costly direct 
coupled units which gain no efficiency over belted combina- 



428 ELECTRIC TRANSMISSION OF POWER. 

li : I 




THE ORGANIZATION OF A POWER STATION. 429 

tions, have little to recommend them save appearance, and pay 
dearly for that. 

The best way to avoid such mistakes is to put aside preju- 
dice and let the makers of generators and prime movers put 
their heads together in consultation and work out the problem 
together. Both are usually anxious to do good work, and will 
arrive at a judicious conclusion. 

Alternating work is sometimes difficult in this respect on 
account of the requirements of frequency, but at the present 
time all the large makers of hydraulic and electrical machinery 
have a sufficient line of patterns to meet most cases easily 
without involving special work to any considerable extent. 

In deciding on the number of units to be employed several 
things must be taken into account. The number should not 
be so small that the temporary crippling of a single unit will 
interfere seriously with the work of the plant. This deter- 
mines the maximum permissible size of each unit. The 
nearer one can come to this without involving difficulties in 
the way of proper speed or serious specialization, the better. 
It is seldom advisable to install less than three units, while in 
some cases a considerably larger number must be used to suit 
the hydraulic conditions. 

To illustrate this point, suppose we are considering a trans- 
mission of 3,000 KW from a water-power with 16 feet available 
head. One would naturally like to install three 1,000 KW 
generators or four of 750 KW. But trouble is encountered at 
once in the wheels. The 1,000 KW machine should have, say 
1,500 HP available at the wheel, and the 750 KW about 1,100. 
Even assuming at once the use of double turbines the highest 
available speed for an output of 1,500 HP would be about 75 to 
80 revolutions per minute, too low for advantageous direct 
coupling at any ordinary frequency; 1,100 HP can be obtained 
at a speed perhaps 10 revolutions per minute higher — not 
enough to be of much service. It is a choice between evils at 
best, either generators of speed so low as to be both expensive 
and difficult to get up to normal efficiency, or belting, when one 
would much prefer to couple direct. At lower heads, say 12 
feet, one would be driven from direct connection; at 30 to 40 
feet head it would be comparatively easy. In the case in hand 



430 ELECTRIC TRANSMISSION OF POWER. 

we are near the dividing line, and it would require very close 
figuring to get at the real facts, figuring which would have to 
be guided by local conditions. The chances are that six 500 
KW generators at about 125 r. p. m., would give a good 
combination of efficiency and cost. As an alternative, one 
might use 750 KW generators either coupled to 3 water 
wheels or rope driven. Each case of this kind has to be 
worked out on its merits. Since the dynamos cost far more 
than water-wheels for the same capacity, if there is any special- 
izing to be done it is cheaper to do it at the wheels. If, how- 
ever, it proves convenient to change the dynamo speed a trifle, 
most generators can be varied 5 per cent either way without 
encountering any difficulties. 

Now and then it becomes necessary to plan for vertical 
wheel shafts. This, unhappily, is apt to confront one at very 
low heads, and leads to immediate difficulty. Direct coupling 
is usually impracticable since the speed is very low, double 
wheels being out of the question, and even if the dynamo could 
be economically built the support of the revolving element 
would be very troublesome. The usual arrangement is to use 
bevel gears, and this is generally the only practicable course. 

It is desirable in any case to operate each dynamo by its 
own special wheels, to avoid complication. Hence the con- 
siderations which determine the number of dynamos also de- 
fine the number of wheels. It is very seldom expedient to 
use more than a single pair of wheels for driving a single 
generator, on account of difficulties in alignment and regula- 
tion and consequent tendency to work inharmoniously. This 
tendency is stronger in impulse wheels than in turbines, on 
account of the very small volume of water generally employed, 
and consequent hypersensitiveness to small changes in the 
amount, pressure, and direction of the stream. So, usually, a 
single wheel or pair of wheels, equal to the task of handling a 
single generator, may be taken as the hydraulic unit. 

For simplicity and economy one should keep down the 
number of generators to the limit already imposed, except as 
special cases may call for an increase. If the plant is to feed 
several transmission lines it is sometimes best to assign separ- 
ate dynamos to each line for one purpose or another, and this 



THE ORGANIZATION OF A POWER STATION. 431 

may make it necessary to increase the total number. The 
requisite security from accident can be in such cases ob- 
tained by one or two spare units, or by shifting a generator 
from a Hghtly loaded line to a heavily loaded one. In point 
of fact the modern generator is a wonderfully reliable machine, 



g^- 




FlG. 237. 



and it is not unusual to find a machine that has run day and 
night, save for a few hours in the week, for many months with- 
out any reserve behind it. The author saw recently a small 
incandescent machine which had run some hours per day in an 
isolated plant, for fourteen consecutive years without a failure 



432 ELECTRIC TRANSMISSION OF POWER. 

of any kind. During that time the armature had been out of 
its bearings but once, to have the commutator turned down. 

In steam driven plants, as in water-power works, the most 
convenient arrangement of generators is generally side by 
side in a single line. So placed they are easy to take care of, 
and the spare room is more available than when it is irregularly 
disposed. In case water-wheels are the prime movers a water- 
tight bulkhead is generally placed between them and the dyna- 
mos, so that leaks or overflows will be confined to the wheel 
pit, where they can do no harm. Through this bulkhead the 
shafts should pass if the units are directly coupled. In case 
of a belt or rope drive it is frequently convenient to place 
wheels and dynamos on different levels, thus obtaining similar 
security. Fig. 237 shows a well-arranged small plant of this 
sort, driven by a pair of Pelton wheels. The plant is so small 
that both dynamos can be conveniently driven by pulleys on a 
very short extension of the wheel shaft. 

In a larger plant each wheel unit would drive a single dyna- 
mo, and the receiver and wheels with their fittings would 
occupy one-half of the station, while the dynamos would be 
placed in the other half, following the same general plan shown 
in Fig. 237. The main point is to get good foundations for 
the dynamos while keeping them out of reach of stray water. 

In an alternating current station it is advisable to drive the 
exciters from special prime movers, so that a change of speed, 
even momentary, in the main machine may not change the 
exciter voltage and thus make a bad matter worse. This is par- 
ticularly iiecessary in water-power plants, where the governing 
is apt to be none too close or prompt. It is a good thing also 
to have plenty of reserve capacity in the exciters, so as never 
to be caught with insufficient exciting power, even in case of 
accident to one exciter. 

Both wheels and dynamos should be thoroughly accessible, 
and wheel and dynamo rooms must be well lighted, naturally 
and artificially. A dark and slippery wheel pit, without suffi- 
cient space around the wheels, is sure to prove a source of 
annoyance and sometimes of serious delays. It should be 
possible to get at every wheel and its fittings and to work 
around them freely when all the other wheels are in full use. 



THE ORGANIZATION OF A POWER STATION. 433 

Sometimes it is useful to separate wheels by bulkheads, pref- 
erably movable, and there should always be floor space 
enough to stand and work on without putting up temporary 
stagings and loose boards. There should always be electric 
lights ready for use around all the working machinery, arc 
lamps or incandescents as may be most convenient, but plenty 
of them. Around the wheels it may sometimes be necessary 
to use incandescents in marine globes to protect them from 
the water, and to install waterproof flexible cable for the mov- 
able lights. 

As an example of good practice in a plant for heavy power 
transmission, operated by turbines under a moderate head, 
the Folsom, Cal., installation shown in Plate XV is worth 
studying. Fig. 1 shows the general character of the power 
house and its relation to the forebay, penstocks, and tail-race. 
The forebay itself is double, being divided lengthwise by a 
wall, on each side of which are the gates and penstocks for 
two double turbines. The tail-races are four masonry arches 
under the power house, uniting then into a single channel. 
The tubular steel penstocks are 8 feet in diameter, and 
the relief pipes above them, 4 feet in diameter. The gates 
are handled by hydraulic cylinders, like the head gates at the 
dam. It will be observed that the wheel pit is not in the 
power house, but in the clear space between the rear wall of 
the power house and the end wall of the forebay, which like 
the other masonry work in this plant, is of granite blocks. 
The power house itself is a spacious two-story brick structure 
on granite foundations. The lower floor is the dynamo room 
while the upper floor contains the transformer room, storage 
space, and so forth, together with the high tension switch- 
board, the lines from which are shown running out from the 
end of the building. The wheels are 30 inch double horizontal 
turbines of the McCormick type, giving about 1,250 HP per 
pair at 300 revolutions per minute under the available normal 
head of 55 feet. There are, besides, two small single horizon- 
tal wheels for driving the exciters. Each of the main wheel 
units carries on its shaft a 15,000 lb. fly-wheel to steady its 
operation under varying loads. 

The arrangement of the wheels and generators is admirably 



434 ELECTRIC TRANSMISSION OF POWER. 

shown in Fig. 2, Plate XV, from a photograph taken during 
the process of construction. 

This gives a view of one complete unit: generator, coupling, 
governor, turbines, and fly-wheel, and includes also an exciter 
and its wheel, not yet aligned and coupled. Four such main 
units and the two exciters, all placed side by side in a single 
row, make up the plant. 

The generators are three-phase machines, of 750 KW 
capacity, at 60'-^. Each has 24 poles, runs at 300 revolutions 
per minute, and weighs about 30 tons. They are of very low 
inductance, with polyodontal bar-wound armatures designed to 
give normally 800 volts between lines, and to produce a very 
close approximation to a true sinusoidal wave form. They 
are normally intended to run in parallel, although there is 
actually a complete circuit per machine available when wanted. 
The wheels were originally installed with Faesch-Piccard 
governors, which functioned fairly well but were not strong 
enough for the heavy service, and have now been replaced. 

When the heavy apparatus was all in place and connected, 
the arched spaces shown in Fig. 2 were walled up except for 
shaft holes, and the wheel pit permanently separated from 
the dynamo room. From the dynamos the current is taken 
to the low tension switchboard facing the row of generators. 
Thence it passes to the transformer room on the second floor 
of the station. Here is a bank of twelve raising transformers, 
of the air blast substation type largely used in the practice of 
the General Electric Company. These raise the working pres- 
sure to 11,000 volts. At this potential the current passes to the 
high tension switchboard and thence to the line. A second 
switchboard in the transformer room serves to distribute the 
low tension current received from the dynamos. 

The general arrangement of this station is excellent, for the 
installation as made. Were the plant to be worked at higher 
voltage for which the air blast transformers would be inad- 
visable, it would be wise not to put the transformer room in 
a second story, but to locate the oil transformers in a fireproof 
space by themselves. 

The line consists of four complete three-phase circuits each 
of No. B. & S. wire. There are two independent pole lines 




Fig. 1. 




Fig. 2. 



PLATE XV. 



THE ORGANIZATION OF A POWER STATION. 435 

running side by side a few rods apart, constructed of red- 
wood poles 40 feet long. Each pole line carries two circuits 
symmetrically arranged on two cross arms, one circuit being 
on each side of the pole, the wires arranged so as to form an 
equilateral triangle, with an angle downward. One of the 
pole lines carries an extra cross arm a few feet below the 
main circuits, to accommodate the telephone circuit. All wires 
are transposed at frequent intervals to lessen induction. The 
pole line is on the southern side of the American River and 
follows in the main the country roads clear into Sacramento, 
the two lines being on opposite sides of the road. The route 
thus followed is a trifle longer than the actual linear distance, 
but the gain in accessibility more than counterbalaiices the 
extra mile or so of line. The high tension line is carried 
along the river through the northern edge of the city fairly 
into the district of load, and is then terminated in a handsome 
brick substation containing the transformer and dynamo 
rooms and the offices of the company. The distribution system 
is mixed in character owing to the operation of the existing 
railway and lighting loads. 

The main distribution circuit is a three-phase four-wire 
circuit worked at 125 volts between the active wires and the 
neutral. This gives an admirable network for lighting and 
motor work, very economical of copper, easy to wire and to 
operate. All the transformers in the substation are arranged 
for a secondary voltage of 125, 250, or 500 as may be desired, 
so as to be ready for any kind of service. 

This plant first went into operation in July, 1895, and has 
since then been in continuous service day and night. No seri- 
ous trouble has been encountered, the high voltage line has 
performed admirably, and there has been no difficulty due to 
inductance, lack of balance, resonance, or any of the other 
things that used to be feared in connection with long distance 
polyphase work. Furthermore the plant is a success financially 
as well as electrically. Apart from Niagara, which even now 
is only beginning long distance work, it has been one of the 
most valuable of the pioneer plants in establishing confidence 
in power transmission, and in putting the art upon a sub- 
stantial basis. 



436 ELECTRIC TRANSMISSION OF POWER. 

Another fine example of three-phase work, of especial interest 
as being operated under a very exceptionally high head, 
is the plant utilized at Fresno, Cal. Fresno is a flourishing 
city of 15,000 inhabitants at the head of the magnificent 
San Joaquin valley in central California. Like other Cali- 
fomian cities, it has been hampered in its development by 
the very high cost of coal — $8 to $10 per ton in carload lots, 
and some of its active citizens cast about for an available 
water-power to develop electrically. Such a one was found on 
the north fork of the San Joaquin River very nearly 35 miles 
from the city. At a point where this stream flows through a 
narrow canon it was diverted, and the stream was carried in a 
series of flumes and canals winding along the hillsides for 
seven miles to a point where it could be dropped back into the 
river bed, 1,600 feet below. 

At this point an emergency reservoir was formed in a natural 
basin, which by an expenditure of less than $3,000 was devel- 
oped into a pond capable of holding enough reserve water for 
several days' run at full load. 

The minimum flow of the stream is 3,000 cubic feet per 
minute, capable of giving between 6,000 and 7,000 HP off the 
shafts of the water-wheels when fully utilized. In the initial 
plant only a small portion of this power is employed. From 
the head works at the reservoir a pipe line is taken down the 
hillside to the power house. The pipe is 4,100 feet long. At 
the upper end for 400 feet a 24 inch riveted steel pipe is used, 
then lap-welded steel pipe is employed diminishing in diameter 
and increasing in thickness toward the lower end, where it 
is 18 inches in diameter, of five-eighths inch mild steel, and 
terminating in a tubular receiver 30 inches in diameter, of 
three-fourths inch steel. The vertical head is 1,410 feet. This 
corresponds to a pressure of 613 lbs. per square inch, while the 
emergent jet has a spouting velocity of 300 feet per second. 

To withstand and utilize this tremendous velocity unusual 
precautions were necessary. The main Pelton wheels, designed 
for 500 HP at 600 revolutions per minute, have solid steel plate 
centres with hard bronze buckets. Each carries on its shaft a 
steel fly-wheel weighing 3 tons, and 5 feet in diameter. With 
their enormous peripheral speed of over 9,000 feet per minute, 




Fig. 1. 




Fig. 2. 





I 


pi 


V 


■ 


1 


■ 


k- 




p 

f 

r. 

1 


■ 


If 








■ 



Fig. 3. 



PLATE XVI. 



THE ORGANIZATION OF A POWER STATION. 437 

these have a powerful steadying effect on the speed of the 
generators. There are four of these wheels, each directly 
coupled to a 350 KW General Electric three-phase generator, 
giving 700 volts at eO--. There are also two 20 HP Pelton 
wheels, each 20 inches in diameter, and each direct coupled to 
a multipolar exciter. All the wheels are controlled independ- 
ently by Pelton differential governors. 

On the main floor of the power house opposite the generators, 
is the bank of raising transformers. These are of 125 KW 
capacity each, of the ordinary air blast type. Space is 
provided for additional transformers more when the load 
demands them. 

These transformers raise the pressure to 19,000 volts 
between lines and from the high tension section of the switch- 
board the current passes to the transmission line. This con- 
sists of two complete three-phase circuits which can be worked 
together or independently. They are of No. 00 bare copper 
wire carried on special double petticoat porcelain insulators, 
all tested at 27,000 volts alternating pressure. 

The pole line is of 35 foot squared redwood poles set 6 feet 
deep. Each pole carries four cross arms. Three of these at 
the top of the pole are for the transmission circuits. These 
are at present confined to the two upper cross arms, leaving 
space for additional circuits below. A fourth short cross arm 
about 4 feet below the others carries the telephone wires. 

Plate XVI gives a good idea of the general arrangement of 
the Fresno plant. Fig. 1 gives a glimpse of the storage 
reservoir at the upper end of the pipe line. Fig. 2 shows the 
situation of the power house below, which is built of native 
granite on a solid rock foundation, with a wooden roof. It is 
75 X 30 feet in size. The wheel pit is seen running along one 
side of the station just outside the wall, through which pass the 
wheel shafts driving the dynamos inside. In the foreground 
appears the beginning of the transmission line. 

Fig. 3 shows the interior of the power house with the dyna- 
mos and transformers in place and the switchboard at the 
further end of the room. 

In the city of Fresno the transmission lines are taken to a 
substantial brick substation in the centre of the city. Here 



438 ELECTRIC TRANSMISSION OF POWER. 

are situated the reducing transformers and accessory apparatus, 
including two 80-light arc dynamos direct coupled to 60 HP 
induction motors. 

The distribution system is threefold. In the central dis- 
trict of the cit}^ a three-phase four- wire network is employed, 
supplied from three 125 KW reducing transformers, and 
worked at 115 volts between active wires and neutral. For 
the outlying residence region three 75 KW transformers supply 
current at 1,000 volts for use with secondary transformers. 
Finally, for reaching neighboring towns, three 40 KW trans- 
formers feed a 3,000 volt subtransmission system. The oper- 
ation of this plant, like that of the Folsom plant, has been 
highly successful from the start, and the electrical troubles 
that have often been feared on long lines at high voltage have 
been conspicuous by their absence: 

Both these plants represent even to-day, first-class practice 
in general equipment and arrangement, save that the voltages 
of transmission have now become ultra conservative, and 
while differing conditions bring their own necessary modi- 
fications, these examples may be regarded as thoroughly typi- 
cal. They have incidentally demonstrated the thorough prac- 
ticability of general distribution of energy for lighting and 
power by polyphase currents under large commercial condi- 
tions, and at distances great enough to involve all the elec- 
trical difficulties likely to be met at the voltage employed. A 
more recent plant of peculiar interest in some of its engineer- 
ing features is that of the Truckee River General Electric 
Company near Floriston, Cal., shown in Plates XVII and 
XVIII. This plant was erected to supply power to the mines 
of the famous Comstock Lode, where it is used for mining 
hoists, milling and pumping, which had formerly been done 
almost entirely by steam provided by burning pine at $8.50 
to $15 per cord. 

The source of the water-power is the Truckee River, an 
unusually steady stream rising among the snows of the Sierra 
Nevada. At the head works is a timber crib dam about 50 
yards long and only 7 feet in height, serving mainly to back 
the water into a wide, slow running canal a couple of hundred 
yards long, which serves also as a settling pond. Thence the 




'^ 



Fig. 1. 




Fig. 2. 



PLATE XV^II. 




PLATE XVIII. 



THE ORGANIZATION OF A POWER STATION. 439 

water passes through the racks into a timber flume, 10 feet deep 
and 6 feet 8 inches wide inside, the entrance being widened to 
28 feet at the racks and tapered to the normal width in a run 
of 40 feet. 

This timber flume, a portion of which is wefl shown in Plate 
XVII, Fig. 1, winds along the hiflsides for a distance of a httle 
more than a mile and a half. It carries 300 cubic feet of water 
per second at a depth of 6 feet in the flume, the corresponding 
velocity being 7.5 feet per second. This flume is carried on 
heavy timber frames 16 feet between centres, with two inter- 
mediate sets of four posts each. Along the line of the flume 
are two spill gates each in the side of a sand box dropped 
below the bottom of the flume. 

This flume terminates in a timber penstock 36 feet long and 21 
feet wide, furnished with a central bulkhead and strongly stayed 
with iron rods. Back of the penstock a spill flume is carried 
for 200 feet alongside the main flume. From the penstock two 
pipes, taking their water through head gates, run to the wheels. 

These pipes are of redwood staves, hooped with |-inch round 
steel, 6 feet in diameter inside, and 160 feet long. The working 
head is 84.5 feet, and a few feet from the power house the 
wooden pipes are wedged into the steel pipes that lead to the 
wheel-cases. Plate XVII, Fig. 2, shows the power house, pen- 
stocks, pipes, and tail-races. The power house itself is 88 X 31 
feet, of brick, with roof of corrugated galvanized iron, and has 
concrete foundations. 

Plate XVIII shows the arrangement of the wheels and gener- 
ators. The wheel plant consists of two pairs of 27-inch McCor- 
mick horizontal turbines, each pair giving 1,400 HP at 400 
r. p. m. Each pair discharges into a central cast-iron draught 
box continued by a 20-foot draught tube. Each pair of wheels 
is directly coupled to a 750 KW, 500 volt three-phase Westing- 
house generator. But instead of the arrangement shown in 
Plates XV and XVI the shafts of both sets of wheels and of 
the generators are in one straight line, with the wheels at its 
extremities. This gives space for a very solid foundation for 
the generators between the arched tail-races, and if need be the 
generators can be directly coupled together so as to run both 
from a single wheel or as a single unit. Arrangements of this 



440 ELECTHIC TRANSMISSION OP POWER. 

kind may be very freely adopted where the units are few, and 
the plant is not built with probable extensions in mind. Each 
generator has a separate multipolar exciter driven by a small 
separate turbine, and each of these receives the water from the 
case of its main wheel and discharges into the corresponding tail- 
race. Each exciter is of sufficient capacity for both generators. 

Each main pair of wheels is regulated by a Lombard gov- 
ernor, one of which appears in the foreground. But to in- 
sure close regulation an unusual device is installed in connec- 
tion with the governors. The supply pipes are too long and 
the head too high to permit the installation of efficient relief 
pipes, as in the Folsom plant, and the enormous inertia of the 
water in the supply pipes was consequently both an incon- 
venience and a menace. Hence relief was provided by a 
huge balanced Ludlow valve connected with the wheel-case and 
the tail-race. This valve is operated by wire ropes and sheaves, 
so connected with the gate shaft of the wheel that when the 
governor closes the wheel gate it opens the relief valve and 
vice versa, thus keeping the velocity of the water nearly con- 
stant. The effect is closely similar to that obtained with the 
deflecting nozzle used with Pelton wheels, and while it wastes 
water, that is of small moment compared with the necessity 
for regulation. 

The 500-volt current from the generators is raised by oil 
insulated transformers to 22,000 volts for the 33-mile trans- 
mission to Virginia City, Nev., which is the centre of utili- 
zation. The pole line is of square sawed redwood poles 30 feet 
long, 11 inches square at the butt and 7 inches square at the top. 
These poles carry two cross arms on which the two three-phase 
circuits of bare No. 4 B. & S. wire are arranged as usual, forming 
an eciuilateral triangle on each side of the poles. The insula- 
tors are porcelain on oil-treated eucalyptus pins. The poles 
are spaced about 40 to the mile, and carry a couple of brackets 
for the telephone line below the cross arms. The three-phase 
lines are transposed every 144 poles. 

The distribution in Virginia City is at 2,250 volts three- 
phase over a maximum radius of about 2 miles. This plant is 
a good example of recent practice in dealing with moderately 
high heads. The timber flume in particular strikes Eastern 



THE ORGANIZATION OF A POWER STATION. 441 

engineers unfavorably at first, but the irrigation companies of 
the Pacific slope have had many years of experience in that 
sort of construction, and have learned that it is easy, cheap, 
and durable when properly cared for. There are hundreds of 
miles of it used for various purposes in California, and in 
many instances it is the only practicable means of water 
delivery. Altogether this particular plant teaches a useful 
lesson in hydraulic construction, and like those just described 
is a very good example of modern engineering. 

At present long-distance plants are rather the exception, and 
in the natural course of events there must be developed a great 
number of power transmissions at quite moderate distances, 
under ten miles or so. Such plants as regards general organi- 
zation do not possess any special peculiarities. The dynamos, 
however, may often be wound for exceptionally high voltage. 
Dynamos for use with raising transformers should be of 
moderate voltage, not much over 2,000 volts unless the units are 
of immense size, or must furnish local power in addition to 
their regular function. 

At moderate voltage the generators gain in cost per unit of 
output, in simplicity, and in comparative immunity from acci- 
dents. They are also likely to be designed for lower arma- 
ture reaction. Nevertheless, there are many cases in which 
it is advisable to install generators for 5,000 to 12,000 volts 
for the sake of economy and simplicity of plant. In fact, it 
is questionable whether it is ever worth while to use raising 
transformers in work at these very moderate transmission 
voltages. As already indicated, such generators should always 
have stationary armatures, and should, and do, have extraor- 
dinarily good insulation. When installed they are sometimes 
insulated from the foundations with scrupulous care, and 
if direct coupled they may be provided with insulating coup- 
lings. Small high-voltage machines have been supported 
on porcelain insulators. Large generators may be carried on 
hardwood timbers thoroughly treated with insulating material, 
and bolted to the foimdation cap stone. As the art of insula- 
tion has progressed, such precautions have become less and 
less necessary, and at 'the present time generators for 10,000 
volts and more are often installed and successfully used 



442 ELECTRIC TRANSMISSION OF POWER. 

without any such general insulation at all. It is desirable to 
surround such machines with an insulated platform a few inches 
above the floor, and to protect the leads with vulcanite tubes. 
It is well also to shield the terminals so that only one can be 
manipulated at a time when the machine is in action. These 
high-voltage generators have proved to be entirely reliable, do 
not seem to be more subject to accident than other generators, 
and if injured are rather more easily repaired than transformers. 

In all plants employing more than a single generator, — and 
this means nearly all power transmission plants of every kind, 
— the generators should be arranged to run in parallel, and in 
most instances should be so operated regularly. Now and 
then generators may advantageously be operated on separate 
lines, as when these lines must be run under different condi- 
tions of regulation, or when a line must be isolated for the 
purpose of carrying a very severe fluctuating load, but for the 
vast majority of plants these expedients are totally unneces- 
sary, and only complicate the operation of the system without 
any material compensating advantage. 

Plants operated for lighting alone can get along after a 
fashion by shifting load quickly from one machine to another, 
an operation quite familiar to most people who have been cus- 
tomers of such a system; but for the general distribution of 
lights and power this procedure is inadmissible, for it usually 
means stopping some or all of the motors. Moreover, it is a 
clumsy method at best, abandoned long ago by continuous 
current stations, and without any excuse for existence save 
villainously bad generator equipment or incompetence in the 
operation of the station. 

All modern generators of good design are capable of running 
in parallel without the slightest difficulty, provided they have 
somewhere nearJy similar magnetic characteristics and are 
intelligently operated. 

It is inadvisable to attempt running a smooth-core and an 
iron-clad armature in parallel, or two machines which are very 
different in regulation or which give very different wave shape, 
but on the other hand such machines ought not to be installed 
together on general principles. The nearer alike the machines, 
the better they will run in parallel. 



THE ORGANIZATION OF A POWER STATION. 443 

No subject has been oftener a topic of fruitless discussion 
than the paralleUng of alternators. As a matter of fact, any 
two similar alternators will go into parallel and stay there with 
very little difficulty, at least if driven from water-wheels, as is 
nearly always the case in transmission plants. 

High-inductance machines have been supposed to be some- 
what easier to put and work in parallel than those of low 
inductance. They certainly can be thrown together carelessly 
with less likehhood of a large synchronizing current flowing 
between them, but with low-inductance machines a little more 
care, or an inductance temporarily inserted between the 
machines, leads to the same end. 

In throwing two alternators of any kind in parallel, they 
should be in the same phase, running at the same speed and at 
approximately the same voltage. The more nearly these con- 
ditions are fulfilled, the less synchronizing current will flow 
between the machines, and hence the more smoothly will they 
drop together. 

The ordinary arrangement of phase lamps shows the relation 
of both speed and phase with ample exactness. When the 
indicator lamp is pulsating at the rate of one period in four or 
five seconds, it is evident that the relative speeds of the 
machines are very nearly right, and it is quite easy to cut in 
the new machine when its phase is very nearly right. One 
soon gets the swing of the slow pulsations, and can catch the 
middle point of the interval of darkness with great accuracy. 
The pulsations can in fact be easily reduced to a ten-second 
period or even longer. It is, on the whole, best to reverse the 
phase lamp connections so that concordance of phase will be 
marked by the lighting up of the phase lamps. The lamps 
should be of such voltage that they will come merely to a 
bright red when the machines are in phase. This arrange- 
ment averts the possibility of a lamp burning out during 
phasing and giving apparent concordance of phase. This 
accident has actually happened — with spectacular results. In 
large stations special synchronism indicators, of which more 
in the next chapter, frequently replace phase lamps to good 
purpose. It is not a bad idea to provide both as a safety 
precaution. 



444 ELECTRIC TRANSMISSION OF POWER. 

It is obviously necessary that the speeds of the two machines 
should be normally alike, and that the speeds should have a 
certain slight flexibility. When belt-driven from the same 
shaft, the various generators to be put in parallel must be run 
very accurately at the same speed, else one of the belts will 
constantly slip and there will be considerable synchronizing 
current. When properly adjusted, the machines should be so 
closely at speed that the phase lamps will have a period of 
from 20 to 30 seconds. This is not a difficult matter when 
driving from the same shaft. In direct-coupled units, or in 
general those driven from independent prime movers, it is best 
to let one governor do the fine adjustment of speed, the others 
being a little more insensitive. Otherwise the governors are 
likely to fight among themselves and be perpetually see-sawing. 

With respect to equality of voltage, the better the regula- 
tion of the generators in themselves, the more necessary it is to 
have them closely at the same voltage when put into, or when 
running in, parallel. Two generators with bad inherent regu- 
lation will divide the load with approximate equality, even if 
put in parallel with a noticeable difference in voltage, since the 
machine that tends to take the heavier current will promptly 
have its voltage battered down and the tendency corrected — 
at the expense, however, of accurate regulation in the plant. 

With machines of low inductance and good regulation, the 
voltages should be very closely the same before putting into 
parallel, to avoid heavy synchronizing current, and they will 
then divide the load correctly with a very slight adjustment 
of the voltage. If the characteristics of the machines are 
known, as they should be, the voltages can be arranged so 
that they will fall together as accurately as if the added 
machine had been put on an artificial load before parallelizing. 

If these precautions are observed, no difficulty will be experi- 
enced in parallel running, and machines in stations many 
miles apart will work together in perfect harmony. This is 
sometimes necessary in large central station work, when a 
portion of the power is transmitted from a distance and a 
portion generated on the spot. It sometimes happens, too, 
that to obtain the amount of water-power that is desired, it 
must be taken from a group of falls. In point of fact, it is a 



THE ORGANIZATION OF A POWER STATION. 445 

perfectly simple matter to operate a number of transmission 
plants in parallel, rather easier than so to operate the machines 
in a single station. The inductance in a long line acts as an 
electro dynamic buffer. 

The magnitude of the transformer units, when transformers 
are used, should be determined by the same considerations that 
apply to generators, except that questions of speed do not 
have to be considered. The smallest number of transformers 
that it is desirable to use is that number which will permit 
the disuse of a single unit without inconvenience. Above this 
number one must be guided by convenience, but in general 
the fewer units the better, since transformers such as are 
used in large transmission work vary very little in efficiency 
under varying load, and hence there is no considerable gain 
in using small units so as to keep them fully loaded. When 
using large transformers the difference in efficiency between 
full load and half load should be no more than two or three- 
tenths of a per cent, and as a rule the general efficiency can- 
not be sensibly improved by using smaller units. 

In polyphase transmission the transformer unit must be 
taken to include all the phases, so that this unit will usually con- 
sist of two or three allied transformers. In three-phase work 
the circuit can be operated either with two or three trans- 
formers, so that in a measure each transformer group contains 
a reserve of capacity, since, if a transformer fails, the remain- 
ing pair can be coiniected to do nearly two-thirds of the work. 
It is inadvisable, however, to try the resultant mesh on a large 
scale save as an emergency expedient, and the raising and 
reducing transformers should regularly be in groups of three for 
three-phase work, connected star or mesh as occasion requires. 
Very recently combined three-phase transformers have begun 
to come into use, but there has not yet been experience enough 
with them to give them a definite place in the art. For very 
high voltage each phase may have several transformers in 
series, although, since single transformers are now made for 
60,000 volts, such a step is needless unless as an emergency 
measure. 

It is advisable in arranging the transformer plant, to bear 
contingencies in mind. Spare transformers are a good form 



446 ELECTRIC TRANSMISSION OF POWER. 

of insurance. In the station raising transformers alone are 
concerned. These are hkely to be of large capacity and high 
voltage. The individual transformers will very seldom be as 
small as 50 KW, and the voltage is sure to be from 5,000 volts 
upward to 10,000, 20,000, or 30,000 volts, and sometimes even 
more up to 60,000. 

For large transformers, both the air-cooled and the oil- 
insulated types are in common use. The former depend 
wholly on solid insulating material and are cooled by a forced 
blast through the ventilating spaces. The heat is so effectively 
carried off by this means that the output can be readily forced 
without any material loss of efficiency, and these transformers 
are therefore somewhat less expensive than others. For 
work up to 10,000 volts or so they are much used and prove 
very satisfactory. For the higher transmission voltages, the 
oil insulation gives a much larger factor of safety, and is there- 
fore usually preferred. The smaller sizes are often merely 
enclosed in a sheet-iron or cast-iron tank with very deep corru- 
gations to gain radiating surface, and which is filled with pet- 
roleum oil carefully freed from the least residual traces of acid 
and water. 

It is a curious fact that such oil seems sometimes to absorb 
moisture from the air, and may even have to be given a sup- 
plemental drying by blowing air through it. Generally, how- 
ever, the oil furnished for this purpose by the manufacturing 
companies is in good condition if kept in closed tanks or 
barrels, but it is advisable to test for moisture in undertaking 
any large use of it. 

In the '^ self-cooled" transformers just referred to, the radia- 
tion from the case is enough to keep the oil from getting too 
hot, and for sizes up to two or three hundred kilowatts this 
form of cooling is very generally used. For larger units the 
natural circulation of the oil as it is warmed by the coils and 
core is hardly sufficient, and forced cooling has to be employed. 

This is generally accomplished by putting just inside the 
boiler iron case of the transformer a coil of brass pipe through 
which cool water is pumped or allowed to flow. This furnishes 
excellent facilities for cooling, and is the plan very generally 
followed in all the larger station transformers. 



THE ORGANIZATION OF A POWER STATION 447 

Plate VI shows the general appearance of these artificially 
cooled transformers, while Plate XVI, Fig. 3, shows a bank of 
the air-cooled type, installed above a common ventilating flue 
which receives air from a motor blower. Fig. 238 shows the 
appearance of a self-cooled oil transformer for three combined 
phases. 

Although transformer oil has so high a flashing point as to 
be practically non-inflammable under any ordinary provo- 
cation, it may still be a source of danger when in considerable 
quantity, and exposed to great and continued heat. It is 




Fig. 238. 



therefore wise to install oil transformers in such wise as to 
prevent the spread of burning oil in case of serious fire. They 
should, therefore, be isolated from inflammable material, and 
provision should be made for draining off the oil in case of 
necessity. The cases are usually provided with heavy cast- 
iron covers through which the terminals come and which 
protect the oil from access of flames or of air in case of short 
circuits, which, by the way, very rarely ignite the oil. 

It is a good plan to locate the transformers with drainage 
spaces around them and exits through which the oil can harm- 
lessly flow if, from a combination of accidents, it escapes from 



448 ELECTRIC TRANSMISSION OF POWER. 

the transformer case. A sloping concrete floor recessed or 
with low barriers to prevent spreading of oil, and a drainage 
flue opening outside the building, is effective, as is also a large 
drainage flue from the bottom of the case, capable of being 
opened without going too near the transformer, which, of course, 
should be cut out of circuit before attempting to drain it. 

Another plan carried out in the Shawinigan Falls plant is 
to make the transformer cover oil-tight, and to provide it with 
a large pipe extending to a sewer. At the bottom of the case 
is another large pipe connected with the water supply, so that, 
in case of combustion inside, the water may be turned on and 
force out the oil through the top. This avoids access of air, 
and the temporary presence of water is not likely to do much 
additional damage. In fact, transformers have been through 
such fires with astonishingly small injury. 

Air blast transformers also involve some risk, as the blast 
fans any burning insulation, and once started combustion may 
go slowly on long after the blast is shut off. They should 
therefore always be installed on a non-combustible floor. 

In several recent plants the transformers have been placed on 
tracks leading from the individual transformer cells to a conven- 
ient stationary crane, so that if injured they can be rolled out 
and taken down without the necessity of a travelling crane, 
which is objectionable as interfering with the free installation 
of high-voltage wires overhead. With the stationary crane safe 
wiring is much facilitated. 

It is well to surround high-voltage transformers, unless the outer 
cases are grounded, as in the water-cooled type, with an insulated 
platform, and in general high-voltage transformers should be 
treated with extreme respect. The high-voltage leads in particular 
are likely to require pretty close watching where they emerge from 
the case, and should be taken out of the transformer house by a 
simple and direct route. High- voltage transformers are gen- 
erally given a room by themselves, sometimes a separate 
building as in Figs. 234 and 235, but even if, as in many smafl 
stations, they are located in the general apparatus room, they 
should be scrupulously railed off and given the place where 
they will do the least mischief in case of accident. As regards 
the connections employed for the transformers, most American 



THE ORGANIZATION OF A POWER STATION. 449 

plants employ three-phase transmission with a separate trans- 
former for each phase. Whether these or combined three- 
phase transformers are used, there are obviously many possible 
methods of connection, since the primaries and secondaries of 
each group may be either in star or in mesh, and the generator 
may also be in star or in mesh. 

In the star connection the voltage between either wire and 

1 
the neutral point is — y= or about 58 per cent of the working 

voltage between wires, and if the neutral point be grounded the 
maximum voltage between a wire and the ground is limited to 
this amount. The voltage demanded of each transformer is 
therefore reduced, and the strain upon the insulators likewise. 
Operating with a grounded neutral, however, implies a more 
or less serious short circuit of one phase in case of a ground 
elsewhere upon the line, together with a flow of current 
through the earth which may and sometimes does cause seri- 




, 2700 
• 2700 



Fig. 239. 

ous trouble to any grounded telegraph or telephone wires in 
the vicinity. 

With the mesh connection the strains upon the insula- 
tion of transformers and line are higher in the proportion of 

1 . 

—7= : 1, but no single ground causes a short circuit or heavy 

earth currents, and if one transformer of the trio is crippled the 
other two can be connected in resultant mesh so as to deliver 
somewhat more than half the original capacity of the bank. 

An ungrounded star connection is very sensitive to grounds 
and other faults, and the neutral point easily drifts so as to 
greatly disturb the phase relations and voltages. This dis- 
turbance affects the whole system, and may cause dangerous 
rise of potential if the conditions are favorable. 

The sort of thing which may happen is well exemplified in 



450 



ELECTRIC TRANSMISSION OF POWER. 



Fig. 239. Here a generator voltage nominally 1,000 is raised 
to 10,000 volts by a star-mesh combination, and lowered by 
a mesh-star to a nominal 1,000 volts for distribution. The 
raising and lowering star neutral points are grounded, but 
the generator neutral is not. The diagram gives the distri- 
bution of voltages when there is a ground on the low-tension 
side of one raising transformer of which the high-tension coil 
is open. The result evidently might be disastrous, even as 
it is, and such an electro dynamic wrench would very possibly 
provoke resonance or start formidable surging. On the face 
of things such an accident would seem to be very improbable, 
but it might easily happen if one undertook to cut out the 
high-tension side of a damaged transformer during the prog- 
ress of a burn-out in the coils.* 

On account of such possibilities all three phases should be 
opened and closed simultaneously and never by single switches, 
unless in changing connections when all the lines are dead. 
As regards the possible abnormalities of voltage due to accident, 
the whole matter may be summed up by saying that the 
regular mesh system is safe from them, and star connections 
are also safe when grounded at the neutrals throughout the 
system including the generator. Mixed connections of star and 
mesh are likewise safe when grounded at the neutrals of every 
star. The following combinations are commonly found in 
practice. 

Connections Through System. 





Raising 


Raising 


Reducing 


Reducing 


Generator. 


Transmission. 


Transmission. 


Transmission. 


Transmission. 




(Low tension.) 


(High tension.) 


(High tension.) 


(Low tension.) 


Mesh 


Mesh 


Mesh 


Mesh 


Mesh 


Mesh 


Mesh 


Mesh 


Mesh 


Star 


Star 


Star 


Star 


Star 


Star 


Mesh 


Mesh 


Star 


Star 


Mesh 


Mesh 


Mesh 


Star 


Star 


Star 



All these can be made thoroughly operative if all the neutrals 
indicated are grounded. The first and fourth on the list are 
perhaps rather more used than the others. The last three 

* For valuable information along this line, see Peck. Trans, A. I. E. E., 
Vol. XX, p. 1248. 



THE ORGANIZATION OF A POWER STATION. 451 

have a star connection on the main transmission circuit, 
which considerably lessens the strain on the insulation, and 
somewhat simplifies protection against lightning and static 
disturbances, but at the cost of heavy short-circuiting in case 
of a ground. Choice of a system depends very much on the 
particular kind of risks likely to be locally met, and in many 
plants the various connections are used just as occasion dictates. 
The real question involved is the desirability of working with 
a grounded neutral, which varies very much according to cir- 
cumstances. In many cases it is advantageously used on a 
large scale, while now and then the conditions seem decidedly 
adverse. 

It should of course be understood that transformers in power 
transmission work can be, and very often are, worked in par- 
allel with the greatest facility. Transformers to be so used 
must have closely similar magnetic characteristics, and par- 
ticularly must regulate alike under varying loads. They must 
also have independent fuses or other safety devices, so that 
each can take care of itself. In all cases it is highly desir- 
able to have one or more spare transformers, ready to be cut 
in at a moment's notice anywhere that may be necessary. 

Where transportation is difficult, the installation of trans- 
formers is rather a serious problem. Generally speaking, it is 
best to sectionalize the coils, each section being independent 
and fully insulated. The core plates can then be taken in in 
bundles and the transformers built up on the spot, with what- 
ever additional insulation may be necessary. Of course 
means must be at hand for the final testing, including a small 
testing transformer to obtain the necessary voltage. 

The most important accessories of a plant pertain to the 
switchboard, which in high-voltage transmission work has to 
be planned and constructed with extraordinary care. The 
component apparatus will be considered in the next chapter. 

The location of the board involves some troublesome con- 
siderations. In small plants by far the best place for it is at 
the general level of the generators and midway the power- 
house wall opposite them. In plants with only three or four 
generators it can sometimes well be placed at one end of the 
building, as in Plate XVI, Fig. 3. Generally speaking, the best 



452 ELECTRIC TRANSMISSION OF POWER. 

location is the most accessible, for in case of trouble it is usually 
necessary to reach the switchboard on the instant. Hence 
it should be close to and easily visible from the line of generators. 
It should also be set so as to have good light both before and 
behind, with plenty of room in the rear. All combustible 
material should be eliminated from its vicinity. 

The modern board is generally built up of marble panels, each 
containing the apparatus for a single generator, with supple- 
mentary panels for the apparatus pertaining to feeders and to 
the plant as a whole. Behind the board are the necessary trans- 
formers for the instruments, all the wiring, and all the high- 
voltage connections. This arrangement implies ample room, 
which is not always allowed for in designing the power 
house. 

In large plants it is now common to install the switchboard 
in an elevated gallery overlooking the generator room, as in 
Fig. 235. This, of course, necessitates a special attendant 
constantly on the watch and alert. It is a very pretty arrange- 
ment when everything is going well, but in case of extremity 
the switchman cannot either see or hear as well as if he were 
nearer the seat of trouble, and it necessitates a great de^l of 
heavy wiring, and high-voltage concealed wiring at that, which 
is a source of some danger. On the whole, it seems inadvis- 
able to use the gallery switchboard unless one is prepared at 
the same time to use a complete system of remote control 
switches, reducing the elevated board to a mere control desk 
overlooking and facing the generators. If the switches are 
also arranged for manual control in emergencies, such an 
arrangement has much to commend it, but as a rule a board of 
the moderate degree of complexity usual to power transmission 
plants is none the better for being in a relatively inaccessible 
gallery. 

Whether the board is an elevated control board or a manual 
board on the floor, there are certain often neglected precautions 
which should be insisted upon. Whatever other switches are 
installed, each generator should be equipped with a switch 
between it and the general connections of the board, as near 
the generator as possible in fact, and able to break the cir- 
cuit under the severest conditions. This should have both 



THE ORGANIZATION OF A POWER STATION. 458 

manual and remote control, if for large output. There have 
been a great many costly accidents in power houses because 
somebody wanted a compact and handsome board, which 
eventually short-circuited inside the switches. 

The present tendency is to do as much of the switching 
work as possible on the low-tension wiring, and to leave the 
high-tension side of the transformers pretty much to itself. 
The tendency is a healthy one, but in many cases there must 
be provision for opening the high-tension circuits under load. 
Switches for such work are readily available at least up to 50,000 
or 60,000 volts. The wiring of a transmission plant should be 
kept as simple as feasible. In so far as is possible, all the main 
leads should be kept in full view, and when they must pass 
out of view they should be insulated with extreme care, and 
preferably carried in separate ducts which can be kept dry and 
clean. One of the very common sources of trouble is found 
in the cables, which some one with an obsession of neatness 
has stored away too compactly. The worst shut-down which 
has occurred in the great Niagara plant since it went into 
operation was due to this cause. Cables running under the 
floor to the switchboard are fertile sources of trouble and 
should be avoided when possible. 

In any event, the high-voltage wires should be in plain sight 
all the way from the transformers to the exit from the build- 
ing. It is far better to do without a permanent travelling 
crane in handling the transformers, than to take the chances 
that come with concealed high-voltage wires. In the Sb«,w- 
inigan Falls plant already referred to, the transformers are 
arranged so that they can be slid upon rails under a fixed 
tackle, and a little ingenuity will usually make it possible to 
locate the high- voltage wires and preferably the generator 
leads also where they shall be in full sight. 

All these things must be taken into account in building a 
power station, since afterthoughts are apt to be costly and 
ineffective. In designing a power transmission plant, every- 
thing about the station should give way to utility, and the 
aim of the designer should be to produce a building that shall 
be convenient, accessible in every part, well lighted, and fire- 
proof. If at the same time it is cheap to construct and of 



454 ELECTRIC TRANSMISSION OF POWER. 

pleasing exterior, so much the better, but stations are not 
intended for decorative purposes. 

In the way of mechanical fittings, the first place is generally 
given to a travelling crane, capacious enough to move every- 
thing which is likely to need moving about the plant. Not 
only is it exceedingly useful in installation, but it may be 
needed for repairs, and in such case may save much valuable 
time. It need not be of the most elaborate construction, 
being only intended for occasional use, and, in view of possible 
interference with the wiring, may sometimes well be reduced 
to the simplest possible terms, merely a bridge to which 
tackle can be affixed when needful. 

It is very important to have at least one man about the 
plant who is a good practical mechanic, and to provide a work- 
room and tool equipment enough to enable small repairs to 
be made on the spot. In most cases material and tools for 
minor electrical repairs are necessary, and they are always 
desirable, for they make it possible to forestall further repairs, 
and often will tide over an emergency, even if outside help 
has finally to be called in. The more isolated the station, the 
more necessary it is to make such provisions, and the more 
spare parts must be at hand. Of hne material there should 
always be plenty in stock to repair breaks, and this stock 
should never be allowed to get low. 

Finally, as regards attendance, incompetent men are dear at 
any price. It pays to employ skilled men and to make it 
worth their while to settle down to permanent work. They 
are valuable all the time, and can be depended upon in an 
emergency when less competent ones would fail. In this as 
in other things, avoid the fault stigmatized in the vernacular 
as "saving at the tap and spilling at the bung-hole." 



CHAPTER XII. 

AUXILIARY AND SWITCHBOARD APPARATUS. 

In this category one may properly place a wide variety of 
apparatus employed in generating and substations for all 
sorts of purposes. A station implies far more than generators 
and prime movers, although the choice and placing of these 
with relation to the work to be done is the chief consideration 
in station design. 

After the generators the most important items in station 
design are the exciter equipment and switchboard, subjects 
merely outlined in the previous chapter. As regards the first, 
certainty in operation is the main requisite. In some of the 
earlier plants it was the custom to provide each generator 
with an individual exciter generally belted to a pulley on the 
generator shaft. This plan is objectionable in that any trivial 
failure in the exciter may put the generator out of service, 
unless an additional source of excitation is provided. Grant- 
ing the necessity of such other source, one naturally falls into 
the judicious present practice of providing two or more exciters 
driven from independent prime movers, and each large enough 
to supply, if occasion requires, exciting current for all the 
generators or for a considerable group of them. 

Speaking in general terms, the exciting energy required is 
from 1 to 3 per cent of the full generator output, and as it is 
good policy never to work an exciter very hard, a con- 
siderable margin of capacity is desirable. 

As a rule, it is well to install exciters of moderate speed, 
directly coupled to independent water-wheels in case of hydrau- 
lic stations. The wheels should be provided with first-class 
regulators and installed in such wise that they will not be 
interfered with by any ordinary hydraulic difficulties. Of 
late it is not unusual to find one or more motor-driven exciters, 
a motor generator set or sets being installed in addition to 
the wheel-driven equipment. The motors are induction mo- 

455 



456 ELECTRIC TRANSMISSION OF POWER. 

tors designed for very small variation of speed with load, and 
supplied with current from the general bus bars of the system. 

This practice has both good and bad features. Its strong point 
is that in case of hydraulic troubles, anchor ice for instance, 
the small wheels may be considerably affected, making it very 
difficult to hold up the voltage. On the other hand, in case 
of trouble on the lines, one is better off with an independent 
drive for the exciter. In any case, the exciter fields should 
be given a liberal margin of capacity, so that in case of reduced 
speed the voltage can be easily kept up. 

In steam-driven stations the motor-driven exciter is a source 
of some economy, since small steam engines are generally 
uneconomical, while the losses in the motor are comparatively 
small. However driven, the exciters should be so connected 
as to allow them to be interchanged at a moment's notice. 
To facilitate this, one should never rely on a single exciter in 
operation, but should keep a spare exciter ready for action, 
even if it is not actually at speed and running in parallel 
with the one in use. 

The exciter panels on'the switchboard should occupy a prom- 
inent and accessible position, since, if anything goes wrong 
with the exciting circuits, it must be remedied at once. The 
location of the exciters themselves is immaterial, save as 
they should be placed where they can be easily inspected 
and cared for. 

The subject of exciters naturally leads to that of voltage 
regulation. The division of the total regulation between the 
generating plant and the substations is always a somewhat 
dubious matter. On long lines in which the total loss is con- 
siderable, the work is generally divided, the coarse regulation 
of the plant as a whole being done at the power plant, and the 
feeder regulation at the substation. If the power station can 
hold constant voltage at the secondary bus bars of the reduc- 
ing transformers, a long step toward good regulation will have 
been taken. This can be done by hand regulation, but at the 
present time there are several automatic regulators quite 
capable of doing the work with sufficient precision. Several 
forms of automatic compounding have already been considered, 
but the regulators proper are instruments responsive to varia- 



AUXILIARY AND SWITCHBOARD APPARATUS. 457 

tions of voltage from whatever cause, and operating to compen- 
sate for variations of speed as well as of load and power factor. 
They may be worked by pressure wires coming back from the 
load, or by the station secondary voltage compensated for 
variations in load. 

They are essentially voltmeter relays acting on the excita- 
tion of the generators or exciters. One of the best known 
forms is the Chapman regulator, of which the typical connec- 
tions are shown in Fig. 240. The relay, it will be noted, is 



Lamp for 

non-induclivf 

resistance 



Series Transformer 





Fig. 240. 



compensated by a variable winding carrying current from a 
series transformer, which enables the voltage to be held con- 
stant, through to the end of the line, and to, or even beyond, 
the reducing secondaries if necessary. The main automatic 
rheostat is controlled by this relay, and is placed ordinarily 
in the field circuit of the generator, or in the field of the exciter 
if the load variations are not likely to be extreme. It is very 
prompt in action, and nearly dead beat. 

The relay is ordinarily adjusted to hold the equivalent 
secondary voltage constant to about one-half of one per cent, 



458 



ELECTRIC TRANSMISSION OF POWER. 



or closer if necessary, and the whole arrangement is simple and 
effective. 

Another excellent and very ingenious voltage regulator is 
made by the General Electric Co. Its connections are shown 
in diagram in Fig. 241 as applied to a single generator and its 
exciter. The fundamental method employed is the opening 



\ 



Spring 



Main Contacts 



A. C. Controlff 
Magnet^ '^ 



It 



Exciter 
Rheostat 




Counter 

Weight 



Itelay Contacts - 



Pivot -^ 






Relay 



^fa^ 



Potential ^ 
Transformer 




Generator Rheostat 



^^ 



s o 



fA/wwf^ Uvwvwi Current 

-r Transformer 

4 — A^A <>- 



A. C. Geaerator 



»vywv> 
WVV 



Fig. 241. 



and closing of a fixed shunt around the field of the exciter. 
At first thought, this would seem to vary the excitation by 
leaps, but the play of the relays keeps the magnetization steady 
by catching it before it has gone further than needful, the 
natural inertia of the magnetic changes giving the requisite 
amount of stability to the process. The d.c. control magnet 
checks too extensive changes without throwing the work on 
the voltage relay proper, which is provided with an adjust- 



AUXILIARY AND SWITCHBOARD APPARATUS. 459 

able compensating winding derived from a series transformer. 
The office of the condenser is to refieve sparking at the main 
contacts. 

Both these regulators give excellent results in practice and 
have proved thoroughly reliable. They take care of both 
variations in speed and in load, but under extreme variations 
of power factor the series windings on the relays may require 
some readjustment. They can be applied to many problems of 
automatic regulation with admirable results. In somewhat 
simpler forms they have been considerably used in regulating 
direct current generators, particularly when driven from water- 
wheels, in which case the effect of small changes of speed may 
have to be guarded against. Their use is extending, although 
many large plants depend entirely upon hand regulation, 
which, if carefully carried out, gives first-class results when the 
load does not vary too erratically. 

Some general suggestions on switchboards have already 
been given, in addition to which it is worth while to examine 
the principles which imderlie switchboard design. 

The whole purpose of a switchboard is to make easy the 
changes in connections necessary in the practical operation 
of a station. It is not to supply architectural effects in polished 
brass and marble, or to furnish employment for extra attend- 
ants. The fundamental switching operations are compara- 
tively simple, and beyond these there are some which are 
desirable and others which are of more or less fanciful value. 
Likewise, in the matter of switchboard instruments, some are 
necessary, others desirable, and still others mere casual conven- 
iences. In the interest of economy and easy operation, it is 
well to keep the design simple, for it is a perfectly easy matter 
to double the necessary cost of a board without making any 
compensating gains. 

The first thing for which provision must be made is the 
connection of the several generators to the bus bars. The 
next is for the connection of these to the lines or to the low- 
tension sides of the several transformers. If the latter, the 
third requisite is the connection of the high-tension sides of the 
transformers to the several lines. As a general rule, the less 
switching done at high tension the better, but it is at times 



460 



ELECTRIC TRANSMISSION OF POWER. 



necessary. Beyond this, provision must be made for the 
excitation of the generators, their operation in parallel, and 
the measurement of their output to guard against overloads. 
Then, in addition, there may be an almost endless array of 
accessories and provisions against more or less remote con- 
tingencies. Fig. 242 gives diagrammatically the elementary 
switching connections for a transmission power station feeding 

III • • ! 



y}/ 



I I 



-t-+ 



/// /// m^ 




' II 



I ! 



I i U-J I i 



-H 



/)(/, /// )(/jB 



I 

I 

i ! 
j ! 
I I 



I 



//< 



)(/ /;/ 



Ex 



/fVf-^ 



Fig. 242. 

duplicate lines. Here E^ E^ are exciters, G^ G^ G^ the genera- 
tors, T^ T^ T^ banks of transformers, and L^ L^ the lines. 

A is the row of excitation switches, B the generator switches, 
C the low-tension transformer switches, D the high-tension 
transformer switches, and F the high-tension line switches. 
H, I, J, are the generator, transformer, and exciter bus bars 
respectively. 



AUXILIARY AND SWITCHBOARD APPARATUS. 461 

The plant is operated in parallel throughout, and the switch- 
ing requirements are of the simplest kind. The moment 
parallel operation is abandoned and separate generators are 
assigned to separate duties, switchboard complication begins. 
Thus, if one introduces the requirement that the lines L^ L^ 
shall be operated entirely independently of each other, in order 
that the service shall be interchangeable in the station the 
bus bars H, I, must be in duplicate, the switches B, C, D, 
must be double-throw or in duplicate, and the switches F must 
be in duplicate. If still more lines are to be entirely indepen- 
dent, the complication increases at a frightful rate, and can 
only be avoided at the cost of lessened interchangeability. 
As high-tension switching devices are expensive, the expense 
incurred for complete interchangeability may run up to a 
good many thousand dollars, and the case is generally com- 
promised. 

When there are many transformers located at some distance 
from the generators, the connections are often made by dupli- 
cate cables from H to a, low-tension distributing bus bar at 
the transformer board. The main thing is to make the switch- 
ing connections as simple as is consistent with security of 
operation under the required conditions. 

Even in Fig. 242 there are required 11 heavy switches, 5 
of them being for the full line voltage, and in theory any one 
of the 11 may have to open its circuit on an overload. 

In most transmission plants, switching at the high voltage 
under load is avoided as far as possible, and the switches at D, 
and also sometimes at F, are merely disconnecting switches 
often with fuses as a protection against overloads. Such dis- 
connecting switches should always for polyphase circuits 
make a complete break of all phases to lessen danger from 
surging, single-pole switches being reserved for cases safe from 
the necessity of opening under load. 

Fuses for transmission circuits are somewhat troublesome 
at the higher voltages, but serve a useful purpose in an emer- 
gency, by cutting off severe overloads. They are disadvan- 
tageous in that they may open but a single leg of the circuit, 
and that in a way to provoke surging, but they only come into 
play in extreme cases when there is trouble ahead, anyhow. 



462 



ELECTRIC TRANSMISSION OF POWER. 



The types used most are expulsion fuses, enclosed powder 
fuses, and very long wire fuses often enclosed in glass tubes. 
Moderate voltage plants, say up to 20,000 volts, are readily 
safe-guarded by fuses, but at higher pressures more caution 
is necessary. But fuses are so much cheaper than any form 
of high- voltage overload switch yet devised — a few dollars 




Fig. 243. 



as against a few hundred — that they cannot be lightly put 
aside. 

As to switches, the oil break type is by far the most reliable 
for breaking considerable currents at high voltage. The 
arcing power of a high-pressure alternating circuit is so tremen- 
dous that in air switches the gaps must be very long, even to 
a good many feet, to entirely prevent any chance of the arc 



AUXILIARY AND SWITCHBOARD APPARATUS. 463 



holding. Now and then the opening switch may catch the 
circuit nearly at zero current and open with very little disturb- 
ance, although as a rule the effects are somewhat pyrotechnic 
in appearance. The switch breaking under oil on the contrary 
very rarely makes any noticeable disturbance and seldom 
starts severe surging. It is the best means of opening a high- 
voltage circuit, and is almost universally used for the principal 
work of the power house. 

Fig. 243 is a typical oil switch for voltages of 13,000 and 
below. It is a three-pole double break switch of the plunger 
form, shown in the cut with the oil tank in which the contacts 



Source 




Overload Coil 



FlQ. 244. 

are submerged, removed for inspection. The plunger is operated 
by means of a toggle-joint from a hand lever on the front of 
the board, and the example here shown is also fitted with an 
electro-magnetic release which can be operated ^rom a remote 
point, or which, if energized by series transformers in the main 
circuit controlled, can convert the switch into an automatic 
circuit breaker for overloads. Such switches are very prompt 
and certain in their action, and serve admirably for the main 
generator switches, or for line switches at moderate voltage. 
As generator switches the automatic overload device is not 
needed in most instances, as station operators ordinarily pre- 
fer to keep the generators in action through any but very 
severe and prolonged overloads. 



464 



ELECTRIC TRANSMISSION OF POWER. 



Not infrequently, time limit relays are applied to such 
automatic switches, arranged with an adjustable dash pot so 
tha,t an overload lasting less than a predetermined number of 
seconds will not cause the opening of the switch. In still 
another modification, this limit is automatically shortened in 
case of very severe overloads. The ordinary connections of 
Fig. 243 as an automatic switch are given by Fig. 244. 

At the present time there is a rather strong tendency, often 
carried to an extreme, to arrange these important switches for 
remote control, the switches being located back of or otherwise 
away from the board and operated by a small actuating switch 







i . 



Fig. 245. 

on the board. This has been a natural outcome of placing 
large and complicated boards in a gallery with limited room 
at the immediate back of the board. It is often inconvenient 
to place large oil switches on the board itself, and it is not a 
bad plan to connect them to the board by operating levers 
which will allow the switches to be placed where they have 
ample room, as in Fig. 245. 

Less commonly, the conditions call for a switch entirely 
operated by electric power. Such switches are generally those 
for very high voltage or very large output, which are bulky 
and require elaborate insulation. They are intended to be 



AUXILIARY AND SWITCHBOARD APPARATUS. 4:65 




Fig. 246. 

operated from the board or by hand if necessary, and are 
placed in a safe and convenient position quite irrespective of 
the board. 

Such a remotely controlled switch as made by the General 
Electric Company, for voltages even up to 60,000 and at 



466 



ELECTRIC TRANSMISSION OF POWER. 



lower voltage for very large outputs, is shown in Fig. 246. It 
is a three-pole double break affair, with each break in a sepa- 
rate oil tank, and each phase in a separate brick compart- 
ment. It is operated by a d.c. series motor generally worked 
from the exciting circuit and controlled from the switchboard 
to which its operation is signalled back. Fig. 247 shows the 
connections of this form of apparatus, which is used chiefly 



Red Indicaline Lamp 
^ (on Switch Closed) 



Closing; Contact 



I < — Opening Contact ' 

]< Green Indicating Lamp 

(Oil Switch Open) 




Automatic Contact Fingers 
Cam Actuated 
Oil Switch in Closed Position 

Fig. 247. 



for very heavy work. One great advantage of remote con- 
trolled switches, seldom, however, realized, is for control of 
the individual generators as at B, Fig. 242. By going to 
remote control it is easy to locate the switches right at the 
generators, so that no trouble at the board can short-circuit 
the generator inside the switches. This accident has happened 
a good many times with disastrous results, for the switch- 
board and its connecting cables are by no means an insignifi- 
cant source of danger. 

In Plate XIX is shown a Westinghouse remote control switch 
for large work at 60,000 volts. It is operated not by a motor 
as in Fig. 246, but by powerful solenoids actuating the switch 
mechanism directly. Switches for such high voltage should 



AUXILIARY AND SWITCHBOARD APPARATUS. 467 

be operated very carefully, and the leads to and from them 
must on account of the voltage be elaborately insulated and 
located with extreme caution. By the use of electrically 
operated switches, it is sometimes possible to simplify the 
high-tension wiring very considerably. They can obviously be 
made automatic if necessary or desirable. They are, however, 
very expensive, and on this account should be used only where 
there is very good cause for choosing them as against hand- 
operated switches. Electric actuation in itself is an additional 
complication, rendering switching easier but somewhat less 
direct and certain, and should be resorted to only when the 





|^:==«..:^:4- 



Fig. 248. 



total complication is thereby materially diminished. In 
installing high-voltage oil switches, they should be provided 
at some point with disconnecting switches to facilitate inspec- 
tion and repairs by cutting them clear from the circuits. 

A capital air switch for disconnecting duty and for opening 
lines under moderate load was described recently by Professor 
Baum, as in successful use up to 60,000 volts by the California 
Gas and Electric Co. It is a three-blade switch mounted on 
high-tension insulators and arranged for operation by a lever 
and long connecting rods. Fig. 248 shows the detail of a 
single blade. An outdoor switch of similar construction, 
made double break, is shown in Fig. 249. This is used for dis- 
connecting load up to say 1,000 K.W, while the former pattern 
is used for connecting the high-tension side of transformers to 
the bus bars. The mam thing in designing such switches is 



468 ELECTRIC TRANSMISSION OF POWER. 

to give them ample insulation and a long, quick break. In 
the practice of The California Gas and Electric Co., no fuses 
or circuit-breakers are used at the power plant, but the trans- 
formers are fused at the substations. 

In the operation of important stations the tendency is to 
hold on as long as practicable in case of a short circuit on the 
lines, on the chance of the line clearing itself without com- 
pelling a shut-down. Hence automatic safety devices are 
rather sparingly employed. In many respects the working of 
long-distance power transmission plants is peculiar. With 
many miles of lines in circuit, a fault, even if it could be quickly 
located; cannot often be quickly reached. The lines are few in 



^^ 



& ^ ^ 




Fig. 249. 

number and heavily loaded, each supplying energy over a great 
area. From the consumer's standpoint it is quite as bad to 
have a line switch opened as to have a line burned off by 
keeping the switch shut. There is a chance of burning off a 
short-circuiting twig, for instance, before the line itself fails, 
and that chance is usually taken — and wisely. 

The instruments used in power transmission stations are 
kept almost entirely on the low- voltage side of the equipment. 
Those needed for each generator are customarily located on 
a standard panel, and these panels are united into a complete 
section of the switchboard. As the generators are in plants 
using raising transformers seldom for higher voltage than 2,000 
to 2,500, the provision of instruments involves no difficulties. 



AUXILIARY AND SWITCHBOARD APPARATUS. 469 

and the equipment of each panel is about as follows in case of a 
three-phase machine: 

3 Ammeters. 

1 Voltmeter. 

1 Plug for connecting voltmeter to any phase. 

1 Main Switch. 

1 Plug for connecting synchronizing device. 

1 Disconnecting Switch for isolating main switch. 

1 Field Rheostat. 

1 Field Switch. 

1 Field Ammeter. 
To this list is sometimes added a wattmeter, either indicat- 
ing or integrating, as the case may be. In assembling these 
into a generator board, there are added the exciter panels, 
each carrying a voltmeter, ammeter, field rheostat, and main 
switch, and in case of motor-driven exciters extra motor panels 
with the appropriate instruments. The necessary potential 
transformers, current transformers, oil switches, and heavy 
accessories are located behind the board, no high-voltage 
parts being allowed upon the front of it. 

For the whole plant there is provided a suitable synchroniz- 
ing device, and to this may be added a frequency indicator 
and a power factor indicator, both of which are convenient, 
although neither is necessary. Besides the panel carrying 
these, there may be others in large systems providing for 
switching groups of generators upon a general main set of bus 
bars, perhaps itself sectionalized. These latter complications, 
however, are seldom found in power transmission plants 
engaged in ordinary general service. 

At present, the once universal phase lamps are commonly 
used only as auxiliaries, the real work falling upon the "syn- 
chroscope" or ''synchronism indicator" which is far more con- 
venient. Such a device is manufactured both by the Westing- 
house and General Electric companies. The latter form is 
shown in Fig. 250. Externally it consists of a case with a dial 
and pointer. When connected to the bus bars and to the in- 
coming machine, the direction of rotation of the pointer shows 
whether the incoming machine is running too fast or too slow, 
a complete revolution meaning the gain or loss of one cycle. 



470 ELECTRIC TRANSMISSION OF POWER. • 

The amount of displacement therefore indicates the phase angle 
of the incoming machine, and when the pointer is steady at 
zero the machines may be thrown together. Internally the de- 
vice is essentially a pair of rudimentary induction motor fields, 
each energized from one of the machines to be parallelized, and 
acting in opposite directions upon a common armature attached 
to the pointer. In polyphase circuits the arrangement is very 
simple. For application to single phases the fields are con- 




FlG. 250. 

nected as split-phase motors by means of a combination of 
resistance and reactance. Obviously the device can then be 
used with either single- phase or polyphase circuits, and such 
is the usual form of the instrument, which is often mounted 
adjacent to a pair of voltmeters, one on the main circuit and 
the other capable of connection to any machine by means of 
a system of potential plugs. 

In the practice of the Westinghouse Co., a very ingenious 
scheme of automatic synchronization has been worked out, 
whereby at the proper moment a relay closes the actuating 



AUXILIARY AND SWITCHBOARD APPARATUS. 4T1 

circuit of an electrically operated main switch. The arrange- 
ment is shown diagrammatically in Fig. 251 as applied to syn- 
chronizing a rotary converter. The relay scheme is very in- 
genious, being a balanced and slightly damped lever actuated 
by opposed coils from the synchronizing circuits. As syn- 
chronism is approached, the resulting pulls alternate at lower 
and lower frequency and are more effective against the damp- 
ing, until finally, when synchronism is reached, the relay cir- 
cuit closes and the machines are thrown together with the 
utmost precision. In many cases a refinement of this sort is 



Three-Phase Bus Bars 



125 Volts, Direct Current 



Controller Switch ( 



A'*'fomaticSynch'OniBer 



S 



Potential Transformers 

Ftg. 251. 




quite unnecessary, in others it is likely to prove exceedingly 
convenient. 

The power factor indicator is a comparatively recent addition 
to station equipment, but one that is most serviceable in station 
operation. In particular it gives a very valuable check on the 
regulation which we have seen varies greatly with the power 
factor. It also enables one very readily to adjust rotary con- 
verters and synchronous motors for minimum input. It is 
essentially an instruinent for balanced circuits, and is graduated 
to read power factors directly on such circuits. It is essentially 
a differential combination of wattmeter and volt-ampere meter. 

The frequency indicator is very convenient in detecting any 



472 



ELECTRIC TRANSMISSION OF POWER. 



tendency to vary from the normal periodicity. In principle it 
is a voltmeter specialized so as to be hypersensitive to varia- 
tions of frequency, and with a scale graduated to these varia- 
tions while being relatively insensitive as a voltmeter when 
near its rated normal voltage by reason of relatively great 
reactance. It might well be given an extra scale, in stations 
having uniform generators, fitted to read generator speed 
directly. 

Ground detectors used to be a regular part of station equip- 
ment, but as transmission voltages have risen these instruments 




Fig. 252. 



have become more and more difficult safelj^ to apply, so that 
as regards high-voltage circuits the}^ are little used, grounds 
making themselves all too obvious without special instruments. 
Up to 10,000 volts or so, and especially on cable circuits, they 
may be of considerable service. Fig. 252 is one of the common 
forms for a three-phase circuit. It, like most of its class, is an 
electrostatic instrument with an electrometer leaf and pointer 
for each phase. 

Most station instruments are now made with illuminated 



AUXILIARY AND SWITCHBOARD APPARATUS. 478 

dials, and are very frequently put up in edgewise form so as 
to economize space on the switchboards. They should be 
checked by standard instruments at frequent intervals, as 
even the best of them are liable to get out of order occasionally. 

The lighting of a station is a simple enough matter, the 
main consideration being to leave no dark corners. It is good 
policy, whatever the ordinary source of current, to have inde- 
pendent means of throwing part at least of the lights upon an 
exciter circuit so that an accident will not leave the station in 
darkness. 

As already intimated, there is a wide range of possibility in 
supplying stations with instruments, switches, and auxiliary 
equipment generally. With respect to automatic devices of 
various kinds especially there is much room for difference of 
opinion. If too liberally supplied, there may be reached a 
point where the care of them is more onerous than the func- 
tions which they assume. There is also some danger that 
the station staff in depending upon them will lose something 
of alertness. And in any case, it must be remembered that 
even the best automatic devices may go wrong, and that 
manual means of control should always be ready for use if 
necessary, and that without delay. 



CHAPTER XIII. 



THE LINE. 



The line is a very important part of a power transmission 
system, for on its integrity depends the continuity of service 
without which even the most perfect apparatus is commer- 
cially useless. In most cases the customer who uses electrical 
power neither knows the efficiency of his motor nor cares 
much about it, so long as the machine goes steadily along 
without the annoyance and expense of frequent repairs. But 
if the service frequently fails, suspending the operation of all 
his machinery while repairs are being executed, the electric 
motor, so far as he is concerned, is a commercial failure, and 
a nuisance to boot, and no representations of cheap power 
can be of much avail when a single stoppage may cause more 
loss than could be recompensed by free power for a month. 

Modern dynamos and motors of almost every class are 
reasonably efficient and reliable, so that as a rule the line is 
the weakest portion of the system. More particularly is this 
the case when the distance of transmission is great and many 
miles of line must be guarded, inspected, and kept in perfect 
working order. In such long lines, not only is the actual labor 
of maintenance great, but the principal engineering difficulties 
will there be encountered. With apparatus of the character 
even now available, the future of electrical power transmission 
depends in very large measure on the development that takes 
place in the construction, insulation, and maintenance of the 
line, together with the solution of certain electrical problems 
that arise as the line grows longer. It is therefore important 
to go into the matter very carefully, as regards not only the 
general arrangements and the electrical details of the work, 
but with respect to methods of construction. 

We may then with advantage divide our consideration of the 
line into three heads. First, the line in its general relations 
to the plant, considering it merely as a conductor. Second, 
the line as a special problem in engineering. Third, the line 

474 



THE LINE. 475 

as a mechanical structure. Of these heads the first has to 
do with such questions as the proper proportioning of the hne 
as a part of the system, its function as a distributing con- 
ductor, and its bearing on the general efficiency of the plant 
of which it is a part. Next come up for examination the 
electrical difficulties that appear in the line, and finally the 
materials of construction and the methods of applying them. 

One of the first questions that arises in designing a plant 
for the transmission of power, is the character and dimensions 
of the conducting system in their relation to the rest of the 
plant. Efficiency is generally the first thing considered — cost 
comes as a gloomy afterthought; and between these two, good 
service is only too frequently neglected. In taking up a trans- 
mission problem, the layman's first query generally is, ''How 
much power will be lost in the line?" and when the engineer 
answers, ''As much or as little as you please," the subject of 
line design is opened up in its broadest aspect. 

Whenever an electrical current traverses a conductor, there 
is a necessary loss of energy due to the fact that all substances 
have an electrical resistance which has to be overcome. The 
energy so lost is substantially all transformed into heat, which 
goes to raising the temperature of the conductor, and indirectly 
that of surrounding bodies. The facts in the case are put 

E 

in their clearest and most compact form by Ohm's law, C = — • 

R 

This states that the current is numerically equal to the elec- 
tromotive force between the points where the current flows, 
divided by the resistance. Hence, this E. M. F. equals the 
current multiplied by the resistance between these points. 

E = CR. 

This tells us at once the loss in E. M. F. between the ends 
of any line, provided we know the current flowing and the 
resistance of the line. And inasmuch as the energy trans- 
mitted by the same current varies directly with the working 
E. M. F., a comparison of the loss in volts determined as above, 
with the initial E. M. F. applied to the circuit, shows the per- 
centage of energy lost in the line. Obviously its absolute 
amomit in watts is equal to the volts lost, multiplied by the 



476 ELECTRIC TRANSMISSION OF POWER. 

current; i.e., CE, or from the last equation C^R if we prefer to 
reckon in terms of resistance. As the loss of energy varies 
with the square of the current, halving the current would 
divide the absolute loss by four, and the percentage loss by 
two, since the total energy is proportional to the current, the 
E. M. F. being fixed. 

A glance shows that the voltage employed is the determin- 
ing factor in the cost of the lines. For a fixed percentage of 
voltage loss, doubling the working voltage will evidently divide 
the amount of copper required by four, since the current for 
a given amount of energy will be reduced by one-half, while 
the actual volts lost will be doubled in maintaining the fixed 
percentage. 

So in general the amount of copper required for transmit- 
ting a given amount of energy a given distance at a fixed effi- 
ciency, will vary inversely as the square of the voltage. 

If the distance of transmission is doubled, the area of the 
conductor will evidently have to be doubled also; conse- 
quently, since the length is doubled, the weight of copper will 
be increased four times. That is, for the same energy trans- 
mitted at the same per cent efficiency and the same voltage, 
the weight of copper will increase directly as the square of the 
distance. The advantage and, indeed, necessity of employing 
high voltages for transmissions over any considerable distance 
is obvious. In fact, it will be seen that by increasing the 
voltage in direct proportion to the distance, the weight of 
copper required for a given percentage of loss will be made a 
constant quantity independent of the distance. 

If one were free to go on increasing the voltage indefinitely 
without enormously enhancing the electrical difficulties, power 
transmission would be a simple task, but unfortunately such 
is not the case. With very high voltages we meet difficulties 
both in establishing and maintaining the insulation of the line, 
and in utilizing the power after it is successfully transmitted. 
The specific character of these limitations will be discussed 
later, but enough has been said to render it evident that in 
establishing a power transmission system, both the working 
voltage and the volts lost in the line must be determined with 
great judgment. 



THE LINE. 477 

In the matter of economy in the line, high voltage is desir- 
able — first, last, and always. In systems where the voltage 
undergoes no transformation, its magnitude is somewhat arbi- 
trarily fixed by the practicable voltage which can be employed 
in the various translating devices, motors, lamps, and the like. 
For example, in a system at constant potential wherein incan- 
descent lamps are an important item, 125 volts, or 250 volts 
as an extreme figure, would be the highest pressure advisable 
for the receiving system in the present state of the art ; or in 
certain cases where cheap power can be had, these voltages 
might be doubled, and 220 to 250 volt lamps used on a three- 
wire system. For a direct-current-motor system the corre- 
sponding figure would be 500 to 600 volts or 1,000 to 1,200 
worked three- wire. Similar limitations indicated elsewhere 
will hold for other classes of apparatus. 

When there is a transformation of voltage in the system, 
whether direct or alternating current, so that the line voltage 
is not fixed by that of the translating devices, it is advisable to 
raise the voltage of transmission as high as the existing state 
of the art permits. It must be borne in mind, however, that 
this general rule is subject to modification by circumstances. 
It would be bad economy, for instance, to use very high pres- 
sures and costly insulation for a transmission of moderate 
length and trifling magnitude. Such practice would result in 
sending perhaps 100 KW over a line or through a conduit 
which could as easily serve for ten times the power without 
great additional cost for copper. It is well, however, not to 
stop at half-way measures, but, if transforming devices are to 
be used at all, to go boldly to the highest voltage which experi- 
ence has shown to be safe on the line, or in the generators, if 
only reducing transformers are used. 

For example, in most cases of alternating current work, 
1,000 volts is entirely obsolete; if the line voltage has to be 
reduced at all, it is better to get the advantage of 2,000 to 
12,000 volts on the line; if raising and reducing transformers 
are employed, the latter figure might as well be increased to 
20,000 or 40,000, unless climatic or other special conditions are 
unfavorable. 

It will be seen that, quite aside from engineering details, 



478 ELECTRIC TRANSMISSION OF POWER. 

divers really commercial factors must enter into any final 
decision regarding the voltage to be used. And these com- 
mercial factors are the final arbiters as to the working voltage, 
and even more completely as to the proportion of energy 
which it is desirable to lose in the line. Power transmission 
systems are installed to earn money, not to establish engineer- 
ing theses. 

It is evident, to start with, that whatever the voltage, high 
efficiency of the line and low first cost are in a measure mutu- 
ally exclusive. The former means large conductors, the 
latter small ones; the former delivers a large percentage of 
salable energy, with a high charge for interest on line invest- 
ment; the latter a smaller amount of energy, with a lessened 
interest account against it. At first sight it would seem easy 
to establish a relation between the cost of energy lost on the 
line and the investment in copper which would be required to 
save it, so that one could comfortably figure out the conditions 
of maximum economy. 

In 1881, Lord Kelvin, then Sir William Thomson, attacked 
the problem and propounded a law, known often by his name, 
which put the general principles of the matter in a very clear 
light, but which indirectly has been responsible for not a little 
downright bad engineering. 

He stated, in effect, that the most economical area of con- 
ductor will be that for which the annual interest charge equals 
the annual cost of energy lost in it. 

While it is true that for a given current and line, Kelvin's 
law correctly indicates the condition of minimum cost in trans- 
mitting said current, this law has" often caused trouble when 
misapplied to concrete cases of power transmission, in that 
it omits many of the practical considerations. It involves 
neither the absolute vahie of the working voltage nor the dis- 
tance of transmission, and for long transmissions at moderate 
voltage often gives absurd values for the energy lost. Indeed, 
as it deals directly only with the most economical condition 
for transmitting energy, it quite neglects the amount of energy 
delivered. In fact, one may apply Kelvin's law rigidly to a 
concrete and not impossible case, and find that no energy to 
speak of will be obtained at the end of the line. 



THE LINE. 479 

In other words, Kelvin's law, while a beautifully correct 
solution of a particular problem, is in its original form totally 
inapplicable to most power transmission work. 

Various investigators, notably Forbes, Kapp, and Perrine, 
have made careful and praiseworthy attempts so to modify Kel- 
vin's law as to take account of all the facts; indeed, nearly every 
writer on power transmission has had a shy at the problem. 

Perhaps the commonest attempt at improvement is to follow 
the general line of the original law, but to equate the interest 
charge on copper to the annual vcilue of the power lost; in 
other words, to proportion the line by increasing the copper 
until the annual net value of a horse-power saved in the line 
would be balanced by the interest charge on the copper re- 
quired to save it. This proposition sounds specious enough 
at first hearing. Practically, it produces a line of greater 
first cost than is usually justified. It is evident that the pos- 
session of a little extra power thus saved brings no profit 
unless it can be sold, and in few cases is a plant worked close 
enough to its maximum capacity during the earlier years of 
its existence to render a trifling increase in output of any 
commercial value, especially in the case of transmission from 
water-power. When the plant is worked at a very high cost 
for power, or soon reaches its full capacity, a few horse-power 
saved in the line will be valuable; but far oftener, particularly 
in water-power plants, it would be cheaper to let the addi- 
tional copper wait until the necessity for it actually arises. 
Furthermore, it evidently does not pay to so increase the line 
investment that the last increment of efficiency will bring no 
profit. 

As an example, let us suppose the case of a 1,000 HP trans- 
mission so constituted that the line copper costs $10,000 with 
10 per cent loss of energy in the line, and suppose in addition 
that the net value of 1 HP at the receiving end is $50 per 
annum. It is evident that by decreasing the loss in the line 
to 2^ per cent there would be available 75 additional HP 
worth $3,750 per annum. The cost of this addition to the 
line would be $30,000, on which interest at 6 per cent would 
be $1,800. So long as the plant is not worked up to 90 per 
cent of its maximum capacity of 1,000 HP, there will be a 



480 ELECTRIC TRANSMISSION OF POWER. 

steady charge of $1,800 plus depreciation, if the additional 
copper be installed at the start. A few months' loss at this 
rate would more than cover the labor of reinforcing the line 
when needed, even supposing that installing the additional 
copper at the start would not have involved extra labor in 
construction. 

Various formulae for designing the line so as to secure the 
minimum cost of transmission have been published, derived 
more or less directly from Kelvin's law, and attempting to 
take into account all the various factors involved in line effi- 
ciency. They all contain quantities of very uncertain value, 
and hence are likely to give correspondingly inexact results. 
More than this, they are founded on two serious misconceptions. 

First, they generally give the minimum cost of transmission, 
which is not at all the same thing as the maximum earning 
power on the total investment. Second, however fully they 
take account of existing conditions, the data on which they 
are founded refer to a particular epoch, and are very unre- 
liable guides in designing a permanent plant. 

A few years or even months, may and often do so change the 
conditions as to lead to a totally different result. In the vast 
majority of cases it is impossible to predict with any accuracy 
the average load on a proposed plant, the average price to be 
obtained for power, or the average efficiency of the translating 
devices which will be used. So probable and natural a thing 
as competition from any cause, or adverse legislation, will 
totally change the conditions of economy. 

For these reasons neither Kelvin's law nor any modification 
thereof, is a safe general guide in determining the proper 
allowance for loss of energy in the line. Only in some specific 
cases is such a law conveniently applicable. Each plant has 
to be considered on its merits, and very various conditions are 
likely to determine the line loss in different cases. The com- 
monest cases which arise are as follows, arranged in order of 
their frequency as occurring in American practice. Each case 
requires a somewhat different treatment in the matter of line 
loss, and the whole classification is the result not of a prion 
reasoning, but of the study of a very large number of concrete 
cases, embracing a wide range of circumstances and covering 



THE LINE. 481 

a large proportion of all the power transmission work that has 
been accomplished or proposed in this country. 

Case I. General distribution of power and hght from water- 
power. This includes something like two-thirds of all the power 
transmission enterprises. The cases which have been investi- 
gated by the author have ranged from 100 to 20,000 HP, to be 
transmitted all the way from one to one hundred and fifty miles. 
The market for power and light is usually uncertain, the propor- 
tion of power to light unknown within wide limits, and the total 
amount required only to be determined by future conditions. 
The average load defies even approximate estimation, and as 
a rule even when the general character of the market is most 
carefully investigated little certainty is gained. 

For one without the gift of prophecy the attempt to figure 
the line for such a transmission by following any canonical 
rules for maximum economy is merely the wildest sort of 
guesswork. The safest process is as follows: Assume an 
amoimt of power to be transmitted which can certainly be 
disposed of. Figure the line for an assumed loss of energy at 
full load small enough to insure good and easy regulation, 
which determines the quality of the service, and hence, in 
large measure, its growth. Arrange both power station and 
line with reference to subsequent increase if needed. The 
exact line loss assumed is more a result of trained judgment 
than of formal calculation. It will be in general between 
5 and 15 per cent, for which losses generators can be con- 
veniently regulated. If raising and reducing transformers are 
used the losses of energy in them should be included in the 
estimate for total loss in the line. In this case the loss in the 
line proper should seldom exceed 10 per cent. A loss of less 
than 5 per cent is seldom advisable. 

It should not be forgotten that in an alternating circuit two 
small conductors are generally better than one large one, so 
that the labor of installation often will not be increased by 
waiting for developments before adding to the line. It fre- 
quently happens, too, that it is very necessary to keep down 
the first cost of installation, to lessen the financial burden 
during the early stages of a plant's development. 

Case II. Delivery of a known amount of power from ample 



482 ELECTRIC TRANSMISSION OF POWER. 

water-power. This condition frequently arises in connection 
with manufacturing estabUshments. A water-power is bought 
or leased in toto, and the problem consists of transmitting 
sufficient power for the comparatively fixed needs of the works. 
The total amount is generally not large, seldom more than 
a few hundred horse-power. Under these circumstances the 
plant should be designed for minimum first cost, and any loss 
in the line is permissible that does not lower the efficiency 
enough to force the use of larger sizes of dynamos and water- 
wheels. These sizes almost invariably are near enough to- 
gether to involve no trouble in regulation if the line be thus 
designed. The operating expense becomes practically a fixed 
charge, so that the first cost only need be considered. 

Such plants are increasingly common. A brief trial calcula- 
tion will show at once the conditions of economy and the way 
to meet them. 

Case III. Delivery of a known power from a closely limited 
source. This case resembles the last, except that there is a 
definite limit set for the losses in the system. Instead, then, 
of fixing a loss in the line based on regulation and first cost 
alone, the first necessity is to deliver the required power. 
This may call for a line more expensive than would be indicated 
by an)^ of the formulae for maximum economy, since it is far 
more important to avoid a supplementary steam plant entirely 
than to escape a considerable increase in cost of line. The data 
to be seriously considered are the cost of maintaining such a 
supplementary plant properly capitalized, and the price of the 
additional copper that render it unnecessary. Maximum 
efficiency is here the governing factor. In cases where the 
motive power is rented or derived from steam, formulae like 
Kelvin's may sometimes be convenient. Losses in the line 
will often be as low as 5 per cent, sometimes only 2 or 3. 

Case IV. Distribution of power in known amount and units, 
with or without long-distance transmission, with motive-power 
which, like steam or rented water-power, costs a certain 
amount per horse-power. Here the desideratum is minimum 
cost per HP, and design for this purpose may be carried out 
with fair accuracy. Small line loss is generally desirable 
unless the system is complicated by a long transmission. 



THE LINE. 483 

Such problems usually or often appear as distributions only. 
Where electric motors are in competition with distribution by 
shafting, rope transmission, and the like, 2 to 5 per cent line 
loss may advantageously be used in a trial computation. 

The problem of power transmission may arise in still other 
forms than those just mentioned. Those are, however, the 
commonest types, and are instanced to show how completely the 
point of vicAV has to change when designing plants under vari- 
ous circumstances. The controlling element may be minimum 
first cost, maximum efficiency, minimum cost of transmission, 
or combinations of any one of these, with locally fixed require- 
ments as to one or more of the others, or as to special con- 
ditions quite apart from any of them. 

In very many cases it is absolutely necessary to keep down 
the initial cost, even at a considerable sacrifice in other respects. 
Or economy in a certain direction must be sought, even at a 
considerable expense in some other direction. For these 
reasons no rigid system can be followed, and there is constant 
necessity for individual skill and judgment. It is no uncom- 
mon thing to find two plants for transmitting equal powers 
over the same distance under very similar conditions, which 
must, however, be installed on totally different plans in order 
to best meet the requirements. 

As regards the general character of transmission lines the 
most usual arrangement is to employ bare copper wire sup- 
ported on wooden or iron poles by suitable insulators. Now 
and then underground construction becomes necessary owing 
to special conditions. Not infrequently an aerial transmission 
line must be coupled with imderground distribution, owing 
to municipal regulations. Occasionally insulated line wire is 
used. It is frequently employed in cases where the transmis- 
sion lines are continued for purposes of distribution through 
the streets of a town, in fact, is usually required. As such 
lines are generally of moderate voltage, very seldom exceed- 
ing 3,000 volts, good standard insulation may often be effec- 
tive in lessening the danger to life in case of accidental contacts, 
and in reducing the trouble from crossing of the lines with 
other lines, branches of trees, and the like. In case of really 
high voltages, 10,000 and upward, no practicable insulation 



484 ELECTRIC TRANSMISSION OF POWER. 

can be trusted for the former purpose, and may in fact create a 
false sense of security, while it is far better practice to endeavor 
to avert the danger of short circuits than to take extraordi- 
nary precautions to mitigate their momentary severity. Hence 
bare copper is to be preferred both on the score of safety and 
of economy. Now and then at some particular point a high 
grade of insulation may minimize local difficulties. 

Much can be said in favor of placing a transmission line 
underground, but there are also very strong reasons against it. 
Such a line is eminently safe, and free from danger of acci- 
dental injury. At the same time it is very difficult to insulate 
properly, and if trouble does arise it is exceedingly hard to 
locate and difficult to remedy. In addition, there are serious 
electrical difficulties to be encountered, which often can be 
reduced only by very costly construction. The chief objec- 
tion aside from these is the expense, which in very many cases 
would be simply prohibitive. 

In cities there is an increasing tendency on the part of the 
authorities to demand underground construction. Overhead 
wires are objectionable on account of their appearance, danger 
to persons and property, and their great inconvenience in 
cases of fire, and these objections apply with almost equal 
total force to all such wires, whether used for electric light or 
power, or for telegraphic and telephonic purposes, the latter 
more than making up by their number for any intrinsic advan- 
tage in the matter of safety. The future city will have its 
electric service completely underground, at least in the more 
densely inhabited portions. It must be said, however, that it 
is far more important for a city to have electric light and 
power than to insist on having it in a particular way, and 
unless the service is very dense, so as to abimdantly justify 
the very great added cost of underground work, private 
capital will hesitate to embark in an enterprise so financially 
overloaded. 

Fortunately, for city distribution moderate voltages must be 
employed on account of the intrinsic limits of direct current 
circuits employed for general distribution, and the undesira- 
bility of distributing transformers of moderate size on very 
high pressure alternating circuits. More than 2,000 to 2,500 



THE LINE. 485 

volts, save on arc circuits, can seldom be used advantageously 
in general distribution, and such voltages can be and are suc- 
cessfully insulated without prohibitive expense. They work 
well in practice, and have stood the test of considerable experi- 
ence. Moreover, with proper care the cables employed as 
conductors, when thoroughly protected and inspected, probably 
have a slightly less rate of depreciation than overhead insulated 
lines, and are much less liable to interruption. As the district 
within which underground service is necessary is usually of no 
great extent, the electrical difficulties that are to be dreaded 
in attempting long underground transmissions are not here of 
so serious magnitude. 

For this limited service, then, in districts where both popula- 
tion and service are dense, there is no serious objection to 
underground lines, and many who have used them are decided 
in commending them as on the whole more convenient and 
reliable than aerial lines. Besides, a large proportion of under- 
ground work is done at low voltages, less than 250 volts, with 
which the difficulties of insulation except at joints are really 
trivial. Such work does not belong so much to power trans- 
mission proper as to distribution from centres after the trans- 
mission is accomplished. 

With high voltages and long distances the case is very dif- 
ferent. Not only are the difficulties of insulation great, but 
electrical troubles are introduced of so severe a character as to 
make success very problematical, even in cases where the cost 
alone is not prohibitive. The feat of cable insulation for pres- 
sures as great as 25,000 volts has been accomplished, and this 
limit could probably be exceeded, but the cost of such work is 
necessarily extremely high, and the location and repair of 
faults is troublesome. An overhead line is so much easier to 
insulate and to maintain that nearly all power transmission 
will probably continue to be carried on by this method for some 
time to come, until, indeed, there are revolutionary changes 
in underground work of which we now have no suggestion. 
The possibility of a long interruption of service while a fault 
is found and repaired is too unpleasant a contingency to be 
incurred. Duplicate lines are a natural recourse in such case, 
effective, but very costly. Aerial lines are much cheaper to 



486 



ELECTRIC TRANSMISSION OF POWER. 



duplicate, and the labor of finding and repairing faults is com- 
paratively light. Finally, when it comes to the question of 
really high voltages like those now coming into frequent use, 
say 40,000 volts to 60,000 volts, it must be admitted that in the 
present state of the art of insulation underground cables, if 
possible at all, are absolutely prohibitive in cost. 

For these reasons underground transmission lines should be 
avoided, certainly until we have had a long experience with 
high voltages overhead. 

Throughout the foregoing it has been assumed that the con- 
ducting line is composed of the best quality of commercial 
copper wire. Inasmuch as other materials are occasionally 
proposed, it it worth while saying something about the relative 
properties of certain metals and alloys as conductors. Aside 
from silver, pure copper is intrinsically the best conductor 
among the metals. In fact, it is hard to say that it is not the 
equal of silver. Commercial copper wire is of somewhat vari- 
able conductivity, since this property is profoundly affected by 
very small proportions of certain other substances. An ad- 
mixture, for instance of one-tenth of one per cent of iron 
reduces the conductivity by about 17 per cent. It used to 
be a most difficult matter to procure commercial wire of good 
quality, and in the early days of telegraphy much annoyance 
was experienced on this score. At present the best grades of 
standard copper wire have a conductivity of fully 98 per cent 
that of chemically pure metal, and even this figure is not 



Material. 


Conductivity. 


Strength, Lbs. 


Commercial copper wire 

Good hard-drawn copper 

(1) Silicon bronze 


98-99 

97-98 
97 
95 
80 

59-60 
45 
26 
14 

10-12 


35,000 

60,000-65,000 

63,200 


Magnesium bronze 


73,000 


(2) Silicon bronze 


76,000 


Aluminium 


32,900 


(3) Silicon bronze. 


110,000 
101,000 


Phosphor bronze. 


Iron annealed wire 


55,000 


High carbon steel wire 


120,000-130,000 



hifrequently exceeded. On account of the comparatively low 
tensile strength of copper, ordinarily about 35,000 lbs. per 



THE LINE. 487 

square inch, very vigorous efforts have been made to exploit 
various alloys of copper on the theory that their greater 
strength would more than overbalance the lessened conduc- 
tivity and increased cost, by enabling less frequent supports to 
be employed. Aluminium bronze, silicon bronze, and phos- 
phor bronze have been tried, together with some other alloys 
of a similar character exploited under various trade names. 
The whole matter of high conductivity bronzes has been so 
saturated with humbug that it is very hard indeed to get at 
the facts in the case. Most of them are tin bronzes carrying 
less than 1 per cent of tin, of which even one-tenth per cent 
will raise the tensile strength by more than 40 per cent, lower- 
ing the conductivity, however, more than hard drawing to the 
same tensile strength. Copper which is hard-dra^^^l probably 
has greater tensile strength than any so-called bronze of sim- 
ilar conductivity, from 60,000 to 65,000 lbs. per square inch, 
with an elastic limit of about 40,000 lbs. per square inch and 
a resistance less than 3 per cent in excess of that of ordi- 
nary copper. The foregoing table gives the conductivities and 
tensile strengths of some of the various materials used or pro- 
posed for line wire, pure copper being taken as the standard 
at 100 per cent conductivity. 

It is sufficiently evident from this table that where the best 
combination of strength and conductivity is wanted, hard- 
drawn copper is unexcelled. For all o^-dinary line work good 
annealed copper wire is amply strong, and is, besides, easier to 
manipulate than wire of greater hardness. Occasionally, 
where it is desirable to use extra long spans, or excessive 
wind pressure is to be encountered, the hard-drawn wire is 
preferable. Not uncommonly a medium hard-drawn copper 
is used having a tensile strength of about 50,000 lbs. per 
square inch and a conductivity of about 98 per cent. Now 
and then, in crossing rivers or ravines, spans of great length 
are desirable — several hundred yards — and in these cases 
one may advantageously employ silicon or other bronze of 
great tensile strength, or as an alternative, a bearer wire, 
preferably a steel wire cable, carrying the copper conducting 
wire or itself serving as the conductor. Where mechanical 
strains are frequent and severe, bronzes are somewhat more 



488 ELECTRIC TRANSMISSION OE POWER. 

reliable than hard-drawn copper of equal tensile strength, 
since they are homogeneous, while the hard-drawn copper 
owes its increase in tenacity to a hard exterior shell, the core 
of it being substantially like ordinary copper. If the prop- 
erties of this skin may be judged by its proportion of the 
total area of the wire, the tensile strength must rise to nearly 
150,000 lbs. per square inch, with a conductivity lowered 10 
to 15 per cent. 

Compound wires have now and then been used, consisting of 
a steel core with a copper covering, but these are costly and 
no better than hard-drawn copper for line use. Iron alone 
replaces copper to any extent. It is cheaper for equal conduc- 
tivity, but in wire is far less durable, and in rods cannot be 
strung overhead conveniently, while, even were this possible, 
the difficulty of making and maintaining joints is most serious. 
Very recently aluminium has been successfully used as a line 
conductor. At present prices (1905) it is materially cheaper 
than copper for equal conductivity, but its bulk and the diffi- 
culty of making joints are sometimes objectionable. Alu- 
minium has about six-tenths the conductivity of copper, the 
resistance of one mil- foot of pure aluminium wire being 17.6 
ohms at 25° C. Owing to its very low specific gravity its 
conductivity is very high when compared on the basis of 
weight. It has very nearly one-half the weight of copper for 
the same conductivity, to be exact 47 per cent, so that as a 
conductor aluminium wire at 30 cents per pound is a little 
cheaper than copper wire at 15 cents per pound. The tensile 
strength of the aluminium is slightly less than that of copper, 
being a little less than 33,000 lbs. per square inch as a max- 
imum, and hi commercial wire usually between 25,000 and 
30,000, while soft-drawn copper is about 34,000 lbs. Like 
soft copper, the aluminium wire takes permanent set very 
easily, having a very low elastic limit, about 14,000 lbs. per 
square inch, so that at about half its ultimate strength it is 
apt to stretch seriously. Comparing wires of equal conduc- 
tivity the aluminium has absolutely greater strength, since 
its cross section is about 1.64 times that of the corresponding 
copper wire. If, however, the copper be hard drawn, the 
aluminium wire of the same conductivity has only about 60 



THE LINE, 489 

per cent of the strength, but having only half the weight of 
the copper, still retains a slight advantage in relation of weight 
to strength. 

Being somewhat larger, the aluminium wire has a trifle 
greater inductance and capacity than the copper and is more 
exposed to the effect of storms. It has about 1.4 times the 
linear coefficient of expansion of copper, so that there is more 
tendency to sag in hot weather and to draw dangerously taut 
in cold weather. This property has caused some practical 
trouble in aluminium lines, and has to be met by great atten- 
tion to temperature and uniform tension in stringing the wire. 
In practical line construction, aluminium is always now used 
in the form of cables laid up of wire, generally No. 8 to No. 12. 
Such cables show somewhat more tensile strength than solid 
wires of similar area and are very much more reliable. They 
have come to be rather widely used and have given excellent 
results. 

Joints in aluminium wire are, as already indicated, a very 
serious problem. In contact with other metals aluminium is 
attacked electrolytically by almost everything, even zinc. A 
successful soldered joint for aluminium has not yet been pro- 
duced, and in line construction recourse has to be taken to 
mechanical joints. One of the most successful of these is that 
used in several California lines. It consists of an oval alumin- 
ium sleeve, large enough to slip iji the two wire ends side by 
side, and for No. 1 wires about 9 inches long. In making the 
joint the ends of the wires were filed rough, the wires were 
slipped side by side through the sleeve, and then by a special 
tool, sleeve and wires were twisted thi^ough two or three com- 
plete turns. The result was a joint practically as strong as 
the original wire, and electrically good. There is considerable 
danger of electrolytic corrosion in any such mechanical joint, 
and lines exposed to salt fogs would probably suffer rather 
severely in this way, but with care in making, and regular 
inspection, these joints serve the purpose well. Very re- 
cently a process of cold welding a sleeve joint under great 
pressure has given excellent results. 

Altogether it seems clear that aluminium is a most useful 
substitute for copper for transmission lines, and it will cer- 



490 ELECTRIC TRANSMISSION OF POWER. 

tainly be used extensively whenever copper rises to a price 
above 15 to 16 cents per pound for bare wire. Not only is 
the aluminum cheaper in first cost, but its lesser weight 
means a great decrease in cost of freights as well. It cer- 
tainly makes an excellent line when carefully put up, and there 
is no good reason why it should not be freely used whenever 
the price of copper throws the balance of economy in favor of 
aluminium. There have been attempts to improve the strength 
of aluminium wire by alloying it, but as in the case of bronzes 
the gain in strength is at the expense of conductivity. Such 
alloy wire should be very cautiously investigated before use. 

Before taking up the practical task of line calculation it is 
necessary to consider somewhat at length the electrical diffi- 
culties that must be encountered, and which impose limitations 
on our practically achieving many things that in themselves 
are desirable and useful. We have seen already that the secret 
of long distance transmission lies in the successful employ- 
ment of very high voltages, and whatever the character of the 
current employed, the difficulties of insulation constantly con- 
front us. These are of various sorts, for the most part, how- 
ever, those that have to do with supporting the conducting 
line so that there may not be a serious loss of current via the 
earth. Next in practical importance come those involved in 
insulating the conductor as a whole against, first, direct earth 
connections or short circuits in underground service, and 
second, grounds or short circuits, if the line is an aerial one. 

In a very large number of cases no attempt is made to 
insulate the wire itself by a continuous covering, and reliance 
is placed entirely on well-insulated supports. In most high 
voltage lines this is the method employed, partly for economy 
but chiefly because there is well-grounded distrust in the du- 
rability of any practicable continuous covering under varying 
climatic conditions and the constant strain imposed by high 
voltage currents. 

So far as supports go, it is evident that while the individual 
resistance of aay particular one may be very great, the total 
resistance of all those throughout the extent of a long line to 
which they are connected in parallel to the earth, may be low 
enough to entail a very considerable total loss of energy. The 



THE LINE. 491 

possibility of such loss increases directly with the number of 
supports throughout the line. The most obvious way of 
reducing such losses would be to considerably increase the 
distance between supports as in some recent constructions. 
This process evidently cannot go on indefinitely, from mechan- 
ical considerations, and hence the greatest advance can be 
made in reducing the chance of loss in individual supports. 

Most of the present practice consists merely of an exten- 
sion of the methods that were devised for telegraphic work. 
These were quite sufficient for the purpose intended, but are 
inadequate when applied to modern high voltage work. 

The ordinary line consists, then, of poles, bearing on pins of 
wood or metal secured to cross arms, bell-shaped glass or por- 
celain insulators. To grooves on or near the top of these the 
line wire is secured by binding wire. Loss of current to earth 
in a line so constituted takes place in two ways. First, the 
current may pass over the outer surface of the insulator, up 
over the interior surface, thence to the supporting pin and so 
to earth. Second, it may actually puncture the substance of 
the insulator and pass directly to the supporting structure. 

The first source of trouble is the commoner, and depends on 
the nature and extent of the insulating surface, and even more 
on climatic conditions. The second depends on the thickness 
and quality of the insulating wall 'which separates the wire 
from the pin. To avoid leakage an insulator should be so 
designed that the extent of surface shall be as long and narrow 
as practicable; also, this surface must be both initially 
and continuously highly insulating. The first condition is 
met by making an insulator of comparatively small diameter, 
and adding to the length of the path over which leakage must 
take place by placing within the outer bell of the insulator 
one or more similar bells (usually called petticoats). These 
not only help in the way mentioned, but they are likely to 
stay tolerably dry even when the exterior surface is wet, and 
thus help to maintain the insulation. 

A good glass or porcelain insulator made on these general 
lines gives excellent results with ordinarily moderate voltages, 
say up to 5,000 volts. When the insulators are new and clean 
they will quite prevent perceptible leakage; and for the vol- 



492 ELECTRIC TRANSMISSION OF POWER. 

tages mentioned are satisfactory under all ordinary conditions. 
When higher voltages are employed the results may be at first 
good, but they are unlikely to stay so unless the climatic con- 
ditions are exceptionally favorable. Most glass permits a 
certain amount of surface leakage, even when new, although 
generally not enough to be of practical importance, but even 
the best commercial glass weathers when exposed to the 
elements, so that in time the surface becomes slightly rough- 
ened and retains a film of dirt and moisture that is a very 
tolerable conductor. Even while perfectly free from this 
deterioration at first, it is generally hygroscopic, because it is 
in a trifling degree soluble even in rain water, and tends to 
retain a slight amount of moisture. Thus in damp climates 
glass is likely to give trouble when used on a high voltage 
line. As regards temporary fall in insulating properties, a 
searching fog or drizzling rain is much worse in its effects on 
insulators than a sharp shower or even a heavy rain, which 
tends to wash the outer surface free of dirt, and affects the 
comparatively clean interior but little. 

Much cheap porcelain is also hygroscopic owing to the poor 
quality of the glaze, and it has the considerable added dis- 
advantage of depending on this glaze for much of its insulat- 
ing value.* Glass is homogeneous throughout its thickness, 
while porcelain inside the 'glaze is often porous and practically 
without insulating value. Nevertheless, porcelain which is 
thoroughly vitrified, the ordinary glaze being replaced by an 
actual fusing of the surface of the material itself, is decidedly 
preferable to ordinary glass, being tough and strong, quite 
non-hygroscopic, and of very high insulating properties. 
The surface does not weather, and the insulation is well kept 
up under all sorts of conditions. If the vitrification extends, 
as it should, considerably below the surface, the insulator will 
resist not only leakage, but puncture, better than any glass. 
The process of making this quality of porcelain is somewhat 
costly, since the baking has to be at an enormous temperature 

* Much American porcelain will absorb 1 to 2 per cent of its weight of 
water, a sign of poor insulating properties. The best porcelain should 
absorb no water and should show a brilliant vitreous fracture which will 
take no flowing stain from ink. 




PLATE XX 



THE LINE. 493 

and long continued, but the result is the most efficient insulat- 
ing substance in use. Glass, however, is better than ordinary 
grades of porcelain. 

Surface discharge is more to be feared than puncture at all 
voltages, since the absolute insulation strength of the material 
can be made high enough, by careful attention to quality and 
sufficient thickness, to withstand any practical voltage contin- 
uously, barring mechanical injury. But leakage is a function 
of moisture, drifting dust, and things meteorological generally, 
besides which, it may take place in serious amount at voltages 
which otherwise would be very easy to work with. 

Up to about 20,000 volts the familiar types of insulator 
of good material and size prove adequate. At higher pressures, 
however, a different state of affairs is encountered, since the 
pressures become sufficient to break down the air as a dielec- 
tric over distances great enough to be inconvenient. 

At about 20,000 volts the lines begin to show a quite per- 
ceptible luminous coating of faint blue at night, little brushes 
spring from the tie wires and sometimes stream from the 
insulators, and as the pressure rises still further these pheno- 
mena become more and more marked. The appearance is 
quite similar to that presented by the high tension leads from 
a large induction coil in a darkened room. 

At 50,000 volts or so the effect is somewhat menacing, and 
unless the lines are well separated there may be considerable 
loss of energy, and it is possible for arcs to strike from wire 
to wire, producing temporary short circuits of most formid- 
able appearance. Plate XX is from a photograph of this 
phenomenon, taken on the lines of the Provo transmission 
where they run through the old basin of the Great Salt Lake. 
A heavy wind will raise clouds of saline dust which is very 
trying to the insulation of the pole tops, and it was during 
such a ''salt-storm" that the picture was taken. Since the 
wires were some six feet between centres, the arc must have 
flamed ten feet high, having been coaxed into action by brush 
discharges over the saline coating of the cross arms. The 
conditions were of course unusual, but at voltages exceeding 
20,000 or 25,000 volts the failure of the air as a dielectric 
introduces an element of difficulty which must be reckoned 



494 



ELECTRIC TRANSMISSION OF POWER. 



with in trying to maintain the insulation of the Hnes. As 
a prehminary to the design of high voltage lines, therefore, 
it is necessary to know approximately the dielectric strength 
of air under practical conditions. This is practically measured 
by the striking distance over which various voltages will 
leap in ordinarily dry air, between sharp points. The strik- 



s 9 



" n - X' "~ ~ " """ ■" ------- 




























/. 
























y _ _ _ - 
























...-..................__...__... _... y ... 




































2 










































iX:: :_:::;:__;::__::::_::::::::::;:::::::__: :;:::::: :_::::_::_::_::_:_:_: 



40 oO 60 70 80 90 lUO 110 

effective sinusoidal voltages in kilovolts 

Fig. 253. 



120 loO 140 150 



ing distances thus taken are greater than between rounded 
surfaces but since the presence of sharp edges and burrs upon 
the line wires or tie wires must be taken into account the 
point distances form a safer guide. 

Fig. 253 shows graphically the relation between effective 
voltage and striking distance, the points used being sharp 
sewing needles. Curve A is from the recent experiments of 



THE LINE. 495 

Fisher*, which were particularly directed to the measurement 
of high voltages by their striking distance, while curve B is 
from the researches of Steinmetz. Below 30,000 volts the 
two are in sufficiently close agreement, but above this point 
large divergences appear in these, and in fact all other experi- 
ments, not too large, however, to make these curves a valu- 
able guide for general purposes. 

In wet air, or at high elevations, and at high temperatures 
to a lesser degree, the striking distances are increased con- 
siderably. Experiments on insulators tested wet by a spray 
and also dry, show that under practical conditions the increase 
due to moisture may be twenty-five or thirty per cent. 

There is, however, sufficient loss of energy and liability Xo 
trouble on high tension lines to make necessary a consider- 
able factor of safety in the aerial insulation strength. The 
brushes and the hissing sound at the insulator's at very high 
pressures speak heavy static discharges and impending trouble, 
even when the air insulation is very thick. In a closed space 
these discharges would quickly so ionize the air as to cause 
discharges, but in the open there is much less danger of this 
occurrence. 

In ordinary practice the diameter of the line wire produces 
very little effect upon the matter of a break down of the air 
although under test conditions in a confined space the size, is 
a very important factor. The reason for this discrepancy 
is very simple — in actual lines the weakest point as regards 
breaking down is at the insulators, and the transmission wires 
on lines long enough to require very high voltage are usually 
for commercial reasons \ inch or more in diameter, and never 
over \ inch except in the case of cables built up of smaller 
wires. 

In other words the practical variation in the radius of the 
wires used is not great, and if they can be made safe at the 
poles there will be little chance of trouble elsewhere. 

The general leakage of a line is the summation of the brush 

leakages at every point. So far as the line wires are concerned 

they are customarily kept far enough apart to avoid direct 

leakage in any amount. The effect of increasing the distance 

* Trans. Int. Elec. Cong. St. Louis, II, 294. 



496 



ELECTRIC TRANSMISSION OF POWER. 



and also the magnitude of the practical energy losses is well 
shown in Fig. 254, which gives the result of tests by Mr. 
Mershon on one of the early lines at Telluride, Colo., 2.25 



1 






























„ 


































































































8500 


























































22" 




8000 






























52 




Loss on Circuit with Wires at 

Different Distances. 

frequency 60; Slotted Armature 

Wires 15, 22, 35 and 52 inches 

apart. 






„ 








8500 


























2000 






























































1500 






























































1000 




























/ 




























1 




' 




























// 


/ 


























y 


> 


f 1 


/ 




















^ 


== 




^ 


- 


-^ 







24 28 33 36 40 
Thousands of Volts 
Fig. 254. 



52 56 



miles long. The conspicuous thing is, that after the energy 
loss exceeds say 100 watts per mile, the breaking down of 
the insulation resistance is very rapid indeed. The breaking 
down point is determined by the height of the peak of the 



THE LINE. 491 

voltage wave, so that in further experiments at Telluride it 
was found that sinusoidal voltages showed less tendency to 
break down the line than indicated in these curves. 

If the pole tops are kept safe from flashing across, the free 
wires will take care of themselves and the weakest point in 
the line insulation is at the insulators themselves, granting 
as we may, that one can get glass or porcelain to resist punc- 
ture at pressures far above the highest now practically used. 
It should be added that Mershon's tests were on No. 6 wire 
and ^viih air somewhat rarified by the elevation, the barom- 
eter reading in the neighborhood of 20 inches. It has been 
shown by Ryan* that the barometric height and the tempera- 
ture greatly influence the point at which the air gives way and 
a coronal discharge sets in. From a considerable series of 
tests Ryan has deduced the following formula for the voltage 
E required to start a coronal discharge between two wires of 
radius r in inches, spaced s inches, when the barometer reads 
b inches and the temperature is f F. 

17 94 h /s\ 

This agrees fairly with experimental results on lines and 
applies to wires from No. 4 B & S up. For smaller wires 
Ryan found values of E much lower than the formula indicates, 
possibly for reasons connected with his method of experiment- 
ation, but the cause of the aberrancies is of small practical im- 
portance since wires smaller than No. 4 are very rarely used 
in transmission work. E it must be remembered is not the 
rated voltage but the peak of the voltage wave, and for sinu- 
soidal waves must be divided by v2 to reduce it to rated 
voltage. 

Moisture seems to produce small effect on the critical vol- 
tage which corresponds with the sharp upward turn in the 
curves of Fig. 254, and the main thing practically is to space 
the lines sufficiently to give a liberal factor of safety between 
E and the working voltage. It would hardly be wise to 
aUow a value of E less than double the working pressure, 

♦Trans. A. I. E. E., Feb. 26, 1904. 



498 ELECTRIC TRANSMISSION OF POWER. 

but even so it would certainly be safe so far as coronal dis- 
charge is concerned to work wires of ordinary size up to 
100,000 volts when spaced six feet or so. Stranded cable 
should give slightly lower values for E than solid wire of 
similar size, but whether materially lower is dubious, and so 
far as practical values of s are concerned, the main problem 
is to resist flashing over the insulator surface in one way 
or another to the cross arm. 

As a matter of fact it is found that when such discharges 
take place they do not follow the insulator surfaces, but jump 



Fig. 255. 

the spaces from petticoat to petticoat. For instance in Fig. 
255 which shows in section a glass insulator designed for use 
at 40,000 volts, the air space which serves as a defence against 
break down is the distance A from upper to lower petticoat, 
plus a small distance B to the pin. Insulators fail by this 
direct discharge and not by a creeping discharge along the sur- 
face. High voltage insulators do not much tend to accumu- 
late moisture which is either repelled or dried off pretty effec- 
tively, and in a rather open construction which favors keeping 
the surfaces free from dirt and moisture, the upper surfaces of 



THE LINE. 499 

petticoats must be regarded in damp weather at least as 
fairly conducting, leaving the sparking distances as shown. 

In Fig. 255 the sparking distance neglecting B is about 6.5 
inches, so that turning to Fig. 253 it appears that if there 
is a difference of 40,000 volts from wire to ground, the given 
insulator has a factor of safety of about 2.5. 

Now considering the fact that in wet weather all pole tops 
may be regarded as giving fair surface conductioQ, it is clear 
that the working air insulation between two wires of a cir- 
cuit carried on insulators like Fig. 255, has an aggregate thick- 
ness of only 13 inches or so, and that this and not the spacing 
of the wires is the real limitation upon the voltage. The 
insulators are always the weakest points of the line both as 
regards general insulation, and danger of arcing. Without 
going into details of insulator construction which will be 
taken up in the next chapter, it may be said that insulators 
of first-class material and of dimensions that should give a 
factor of safety of not less than 2.5 on a working voltage of 
60,000 are now commercially obtainable. 

This factor of safety is none too large, and when one 
considers that very high voltage renders a line particularly 
liable to interruption from accidents which at moderate vol ■ 
tages would be trivial, it is a wonder that transmission lines 
perform as well as they do. 

As to voltages for such lines great progress has been made. 
10,000 to 15,000 volts is a conservative pressure now used 
only for short distances. A common rough and ready rule 
for voltage is a thousand volts per mile or as near it as you 
dare on the longer distances. The present tendency is to use 
not less than 20,000 to 30,000 volts for all serious transmission 
projects. Such pressures have now been in regular service 
with excellent results for half a dozen years past, and that 
in many cases. It may be fairly said that they may be regarded 
as not only completely reliable, but rather conservative. 
They are in operation in all parts of the country, under all 
sorts of climatic conditions, without experiencing any diffi- 
culties which would not be equally in evidence at half the 
voltage. 

In other words, a line can be built and operated at 20,000 



500 ELECTRIC TRANSMISSION OF POWER. 

to 30,000 volts, without trespassing on entirely safe values of 
the factors of safety in the various parts. 

From 30,000 to 45,000 volts there are now in operation 
more than a score of plants and the reports from them are 
uniformly rather favorable. There is no doubt that certain 
classes of line troubles become more prominent, especially in 
reaching the neighborhood of 40,000 volts and above. The 
root of these troubles is the relatively low factor of safety at 
the insulating supports. All may go well under normal con- 
ditions but there is always danger that deterioration or abnor- 
mal pressures arising from one cause or another may break 
down the air gap. If insulators are worked at a voltage which 
is near the sparking distance voltage, dirt, and moisture par- 
ticularly from sea fogs, may so much reduce the surface resis- 
tance as to lead a discharge over and start an arc. It sometimes 
happens that insulators individually tested with a good factor 
of safety will later break down without any adequate electrical 
cause, probably from the starting of cracks from mechanical 
strain. Pins may break or bend thus letting the wire down 
upon or near the cross arm, and many minor faults not con- 
spicuous at 25,000 volts may become serious as the voltage 
nears that at which current will jump the insulators. 

Nevertheless, a good many plants have been working at 
these high pressures with relatively small trouble. Now and 
then temporary shut downs occur, as upon plants at lower 
voltage, but on the whole accidents are few and even these 
are seldom fairly chargeable to the unusual voltage. 

Transmission plants working at 45,000 to 60,000 volts are 
few in number, but are generally of considerable magnitude, 
and have probably been as reliable as those in the class just 
considered. They have had at times trouble with insulators, 
but as they have not temporized with the problem, and have 
used the very best insulators obtainable, they are working 
upon a factor of safety quite as large as that found in many 
plants of much lower voltage, and consequently have not 
experienced unusual difficulties. Certainly several plants are 
doing good commercial work at voltages of at least 60,000. 

Let us now sum up our present knowledge of the transmis- 



THE LINE. 501 

sion of electrical energy over high voltage lines. From a con- 
siderable amount of experience, we are sure that there is no 
real difficulty whatever in establishing and maintaining ade- 
quate insulation up to an effective pressure of 25,000 volts. 
Above this the plants are less numerous, but it is quite 
certain that satisfactory results can regularly be reached up 
to 30,000 without very extraordinary precautions. With 
good climatic conditions 40,000 or 50,000 may be considered 
entirely practicable, with reasonable precautions, and 60,000 
has now been passed without any signs of impending failure. 

At still higher voltages the difficulties are likely to multiply 
more rapidly, and a point will ultimately be reached at which 
the cost of insulating devices will overbalance the saving of 
copper due to increased voltage. This point is at present inde- 
terminate, and will always depend on the amount of power 
to be transmitted, the permissible loss in the line, and un- 
known variables involving repairs and depreciation, cost and 
depreciation of transformers and so on. It is quite impossible 
from present data to set such a limit even approximately, for 
we know as yet nothing of the relative difficulty of insulat- 
ing voltages considerably above the range of our experience. 

In cases where continuous insulation is employed, it is 
for one of two purposes, chiefly to prevent interference with 
the circuit by such accidents as twigs or wires falling across 
the line, and either short circuiting the lines or grounding 
them. Aside from this, the only other object in insulation 
is to lessen the danger to persons accidentally touching the 
wires and to prevent the current straying to other circuits. 

With moderate voltages both these ends can be reached with 
a fair degree of success. With high voltages it is very diffi- 
cult, and in many cases well-nigh impossible. 

Nearly all materials which are available for insulation 
deteriorate to a very marked extent when exposed to the 
weather. Those substances which are the best insulators, 
such as porcelain, glass, mica, and the like, cannot be used 
for continuous insulation, and, in fact, our best insulators 
are mechanically so bad as to be impracticable. There 
is a large class of insulators complicated in chemical consti- 
tution, but mechanically excellent; these are the plastic or 



502 ELECTRIC TRANSMISSION OF POWER. 

semi-plastic substances like gutta-percha, India rubber, 
, bitumen, paraffin, and the like. All of these are subject to 
more or less decomposition, more particularly those which 
are, through good mechanical qualities, desirable for insula- 
tion. All which have been mentioned are sufficiently good 
insulators to answer every practical requirement, if they do 
not deteriorate. 

Gutta-percha and India rubber are decidedly the best of 
these; but gutta-percha is too plastic at anything excepting 
low temperatures to be mechanically good. Gutta-percha ffils, 
however, an unique place on account of its remarkable ability 
to withstand the action of salt water, and it is the most reliable 
insulator for submarine work. For overhead work it is nearly 
useless, as the heat of the sun softens it so as to endanger its 
continuity, and even a moderate increase in temperature may 
decrease its specific resistance to a tenth of its ordinary value. 

India rubber is, by all odds, the best all around insulator 
for overhead lines. In its pure state it deteriorates with very 
great rapidity ; but when vulcanized by the addition of a small 
amount of sulphur, its chemical character is so changed as to 
resist both spontaneous changes and those due to the atmos- 
phere to a very considerable extent, without injury to its 
insulating properties. It is, however, costly, and is eventually 
affected by the weather. To cheapen the manufacture of 
insulated wire a large variety of rubber compounds are em- 
ployed, consisting of mixtures of rubber with various other 
substances intended to give the material good mechanical and 
insulating qualities at less expense. These rubber compounds 
are much inferior to pure vulcanized rubber in point of specific 
resistance, but make a good and substantial covering for 
ordinary purposes, sometimes more durable than the purer 
material. They are very generally employed for commercial 
work. 

Insulated wires for overhead work may be divided into two 
classes. First, those which are so prepared as to withstand 
the weather to a considerable extent and to retain high insu- 
lating properties even in bad weather. Such wires are usually 
covered with compound fairly rich in vulcanized rubber, com- 
monly protected outside with a braiding of cotton saturated 



THE LINE, 503 

with some insulating compound, and serving to protect the 
main insulation from mechanical injury. 

The second class of wires includes those in which no solid 
insulating material is used, but which are thoroughly protected 
by a covering of fibrous material saturated with compounds of 
rubber, bitumen, or the like. These wires are most exten- 
sively used; the insulation is good in dry weather, and fair 
under most ordinary circumstances, but generally greatly in- 
ferior to those wires which are given a coating of rubber. , 

So far as protection of the wire from accidental contacts is 
concerned, either class of insulation is tolerably effective at 
moderate voltages until the covering becomes worn or 
weathered by long or hard usage. 

As regards danger in touching such wires, at moderate 
voltages both kinds of insulation afford a fair degree of pro- 
tection. At high voltages neither can be trusted, in spite of 
the apparently high insulation resistance. There is good 
reason to believe that any insulation employed on Avires is 
greatly affected by the strain of high voltage. Tests made 
with the ordinary Wheatstone bridge give us no useful inform- 
ation as to the action of the same insulation under continued 
stresses of 5,000 or 10,000 volts. Tests made with pressures 
ranging up to even 500 volts show generally a noticeable, 
although very irregular, falling off in resistance, and the 
higher the voltage is carried the more likelihood of complete 
breaking down of the insulation and the more irregular the 
results. 

It is improbable that even the most careful insulation with 
vulcanized rubber of any reasonable thickness would give a 
wire which, under a pressure of 10,000 volts, could be long 
depended on to remove all danger to persons from accidental 
contact. Even if entirely safe at first, it would be unlikely 
to remain so for any great length of time. A rubber covered 
lead sheathed cable with the sheath thoroughly grounded is 
probably the nearest approximation to safety. So serious is 
the difficulty of continuous insulation of high pressures, that 
it is best not seriously to attempt it; but either to fall back 
upon bare wire with very complete insulation at the supports, 
or, if insulated wire be employed at all, to use an insulation 



504 ELECTRIC TRANSMISSION OF POWER. 

intended only to lessen the danger of short circuits from 
falling objects, and always to treat the line, so far as personal 
contact goes, precisely as though it were bare wire. 

Information regarding the insulation of lines, whether of 
bare or insulated wire, under high voltage, is very scarce; but 
all such lines should be treated at all times as if they were 
grounded, in spite of any tests of the insulation that may have 
been made. Theoretically, one should be able to touch a 
completely insulated circuit without danger save from static 
charge; but, practically, it is suicidal so to treat any high 
voltage circuit. 

The writer calls to mind one case in which a man was 
instantly killed, while standing on a dry concrete floor, by 
contact with a 10,000 volt circuit. He probably touched a 
bare portion of the wire, but so far from the general insulation 
of the circuit saving him, the current which he received was 
sufficient to burn into the concrete floor the print of the nails in 
one of his shoes. The ordinary tests on the line made shortly 
afterward showed no particular ground, nor was there any 
reason to believe that one existed at the time of the accident. 
Other accidents, under similar conditions, have occurred with 
arc light circuits of lesser voltage, on which there was a similar 
absence of perceptible ground. It is advisable, therefore, that 
all high voltage circuits should be treated as uninsulated, so far 
as contact is concerned, at all times, and if insulation tests are 
to be made upon them to determine the resistance to ground, 
these tests should be made with, at least, the full voltage of the 
circuit. It is quite as well not to place too much reliance on 
insulation of any kind; but to regard a high voltage electrical 
circuit as dangerous, and to be treated with the same respect as 
is due to other useful, but dangerous, agents, like high pressure 
steam and dynamite, neither of which is likely to be abandoned 
on account of the danger that comes from careless use. The 
precautions taken, either with these or with high voltage cur- 
rents, should be in the direction of preventing such careless- 
ness as might result disastrously. 

An electrical circuit should be so installed that no material 
risk can be run by any person who is not indulging in wilful 
interference with the line, and in such case, if an accident 



THE LINE. 605 

occurs, the victim is deserving of no more sympathy than one 
who dehberately stands in front of an express train. 

If the circuit is of bare wire, there can be no doubt in the 
mind of any one as to its dangerous character, whereas, if 
insulated wire is employed, there is likely to be established a 
certain false sense of security. There is no good reason, 
therefore, for advising the extensive use of insulated wire for 
high voltage lines. 

The ideal overhead circuit is one in which the conductor is 
thoroughly insulated as regards leakage, carefully protected 
from danger of wires or branches falling across it, and placed 
out of the reach of anything except deliberate interference of 
human beings. There may be places at various points along 
the line where insulation would be desirable, in order to avoid 
extensive cutting away of trees, branches of which might fall 
upon the line, or where local regulations require the use of 
insulated wire. Except under these circumstances continuous 
insulation increases the cost and maintenance of the line 
without giving any adequate returns in security. On rare 
occasions, portions of the high voltage circuit may have to be 
placed underground. Here only the very best quality of 
insulation should be employed, thoroughly protected by an 
outside sheathing of lead against the effects of moisture, and 
installed in smooth, clean, dry, and accessible conduits with 
especial attention to insulation at the joints. Of this, more 
in Chapter XIV. 

From what has been said, it should be understood that while 
the problem of installing high voltage lines is unquestionably 
a difficult one, we have not yet had sufficient experience to be 
able to say definitely how difficult it may be. It is very cer- 
tain that much more can be done than has been accomplished. 
It seems probable that so far as overhead work is concerned, 
it will before long be practical to employ voltages considerably 
greater than those now in use. Before any limit can be set 
to the progress in this direction, we need ample experimental 
data, not only on the behavior of insulation at a very high 
pressure, but on the maximum voltage which is likely to be 
encountered when a certain effective voltage is to be employed. 
This opens up a wide field for investigation, involving con- 



606 ELECTRIC TRANSMISSION OF POWER. 

ditions of unknown seriousness, connected especially with the 
electrical peculiarities of alternating currents, which there is 
every reason to believe will be employed almost exclusively on 
high voltage work. 

The special difficulties to be met in working with alternating 
currents are two — inductance in the line and apparatus, and 
electrostatic capacity, accompanied by the very serious phe- 
nomena of electrical resonance. In addition to these, what- 
ever the character of the current used for transmission purposes, 
there is danger of getting accidentally upon the line a voltage 
much higher than the normal. Inductance is met with to a 
very considerable extent in all alternating circuits; resonance 
in a small degree is probably much commoner than is generally 
supposed, and abnormal voltage, due to the generators them- 
selves, must always be guarded against. 

Passing at once to the practical side of the question, we 
find that when an alternating current is sent through any 




£ 



Figs. 256 and 257. 

conductor, it has to deal not only with the electrical resis- 
tance of that wire, but with a virtual resistance due to the 
fact that the electro-magnetic stresses set up at any point 
of the conductor set up electromotive forces at other points 
in the same conductor, which oppose and retard the passage 
of the current. 

These matters have been fully discussed theoretically in 
Chapter IV, and hence will be here but briefly mentioned. 

For example, if a wire be bent into a couple of spiral coils 
like Fig. 256, the electro-magnetic field of one coil will affect the 
other, just as we have induction from one separate ring to 
another in Fig. 4, page 13. If such a spiral has an iron core, 
this self-inductance will be much increased. Even if only a 
straight wire be concerned in the carrying of current, there 
will be a similar inductive relation between the inner and outer 



THE LINE. 507 

portions of the wire at any point, since the electro-magnetic 
stresses exist inside the wire as well as outside. 

Let Fig. 257 represent a circuit carrying an alternating cur- 
rent, which at a given moment is flowing as sho\\Ti by the 
arrows. The electro-magnetic field set up by this current in 
the loop has a direction perpendicular to the plane of the 
paper, and sets up an E. M. F. opposing that of the wire. The 
greater the area of the loop, i.e., the farther apart the two 
wires, the greater proportioQ of the electro-magnetic field will 
pass within the loop and produce self-induction. 

Similarly, the larger the wires for a given distance between 
them, the less effective field within the loop to set up induc- 
tance. In fact, the amount of inductance in the circuit 
depends directly on the ratio between the radii of the wires 
and the distance between them. So if the diameter of the 
wire is decreased to one-half the original amount, the wires 
must be stnmg only half as far apart in order to retain the 
same inductance. 

The practical effect of inductance in the line is to neces- 
sitate the use of an initial E. M. F. large enough to overcome 
the inductive loss of voltage, as well as that due to resistance, 
and so keep the E. M. F. at the receiving end of the line up 
to its proper value. To undertake in an orderly way the 
design of a power transmission line we may consider seriatim 
the effects of resistance, inductance, and capacity as determin- 
ing the losses and the precision of regulation and as related 
to the abnormal values of the voltage which determine the 
real factor of safety in the insulation. 

To begin with. Ohm's law is the basis of all computations 
regarding the line, and lies behind all the formulae used for 
this purpose. The most obvious way of applying it would 
be to find the resistance of the Avhole line corresponding to 
the required current and loss in voltage, and then to look 
up in a wire table the wire which taken of the required length 
would give this resistance. 

As a matter of convenience in computation, various formula) 
have been devised to include in simple form the factors of 
distance, voltage, power transmitted and loss in the line, and 
giving the area weight or cost of the conductors. 



508 ELECTRIC TRANSMISSION OF POWER. 

The area of wires is in English speaking countries expressed 
in terms of the circular mil (cm.), which is the area of a circle 
0.001 inch in diameter, a barbarous unit, which, however, 
by merest chance leads to formulae numerically simple. 

The following formulae are perhaps the most convenient 
of those in use. They are derived as follows. Starting with 

E 

Ohm's law R =- — and remembering that for any wire 

Total length in ft. X resistance of 1 ft. of wire 1 mil in diameter 

xt = : : 

Area in circular mils 

we obtain since the resistance of 1 mil-foot of copper wire is 

very nearly 11 ohms, 

11 L 



R = 



cm. 



or taking the total length of wire as twice the distance of 
transmission in feet, since this distance is the thing immedi- 
ately concerned we have 

2D xU 



R 



cm. 



Now substituting this value of R in the expression for Ohm's 

law we have 

2DxllxC 
cm. = (1) 

This gives the area of the wire for delivering any current 
over any distance with any loss, E in volts. The correspond- 
ing sizes and weights of wire can be looked up in any wire 
table. 

As a matter of convenience the following table gives for 
the sizes of wire likely to be used in power transmission the 
area in circular mils, the diameter, resistance per thousand 
feet, weight per thousand feet bare, and weight also with 
insulation of the so-called weather-proof grade, commonly 
used on distributing circuits. The diameters are given to 
the nearest mil, the areas to the nearest 10 cm. and weights 
to the nearest pound. No wires larger than 0000 are here 
considered, since even this size of copper is seldom used, and 



THE LINE. 



509 



Circular 
Mils. 


Gauge No. 
B. &S. 


Diameter 
in Mils. 


Ohms per M 
ft. at 70° F. 
and 98% Con- 
ductivity. 


Ohms 
per mi]. 


Wt. per M 
feet Bare. 


Wt. per M 

ft. We'th'r 

proof. 


211.600 


0000 


460 


.05026 


.26537 


640 


725 


167,800 


000 


410 


.06337 


.33459 


508 


580 


133,100 


00 


365 


.07991 


.42182 


403 


480 


105.600 





325 


.10077 


.53196 


320 


375 


83.690 


1 


289 


.12707 


.67093 


253 


307 


66,370 


2 


258 


.16024 


.84606 


201 


245 


52,630 


3 


229 


.20206 


1.06687 


159 


195 


41,740 


4 


204 


.25479 


1.34529 


126 


147 


33,100 


5 


182 


.32129 


1.69651 


100 


121 


26,250 


6 


162 


.40516 


2.13924 


79 


99 



on the other hand wires smaller than No. 6 are mechanically 
weak and rarely would be advantageous. In fact the sizes 
No. 00 to No. 2 inclusive include the wires commonly used. 

The actual value of the mil-foot constant at ordinary tem- 
peratures is approximately 10.8, but is here taken as 11 ohms 
to take account of the ordinary contingencies of irregular 
diameter, slight variation in conductivity, and the effect of 
hard drawing. 

The next step in simplifying the computations, is to find a 
simple expression for the weight of the wire required. Now 
it chances that a copper wire 1,000 cm. in area, weighs very 
nearly 3 lbs. per thousand feet, and hence we can get a very 
simple formula giving directly the weight in pounds per thou- 
sand feet. Taking D in thousands of feet and expressing this 
fact by writing it D^^ we have 

2 D^ X 33 X C 

(2) 



T7^ =- 



E 



or for the total weight of the wire 



TF = 



4 i)^^ X 33 X C 
E 



(3) 



This applies to ordinary direct current or single-phase circuits. 
Now we have already seen that each conductor of a three- 
phase line has one-half the area of one wire of the equivalent 
single-phase line, so that by dropping the factor 2 in (1) we 
have for the required area of one wire 



510 ELECTRIC TRANSMISSION OF POWER. 



7)xllx| 
c-m. = •• (4) 

w 
Herein -— is taken to avoid any possible confusion as to the 

value of C, w being the watts at either end of the line and 
V the working voltage at the same end, while E as before 
is the loss in volts. Then proceeding as before we get for 
the total weight of the three-phase lines, 

'''"^4 (5) 

T^ = -__ 

the constant being taken as 100, instead of 99, to compensate 
for a minute deficit, in the assumption of 3 lbs. per thousand 
feet for a wire of 1,000 cm. 

This particular simplification lends itself very readily to 
a cost formula in which P is the total price in dollars when 
p is the price of copper wire taken in cents per pound; as 
follows: ^ 

Finally, since a power factor less than unity implies the deliv- 
ery of increased current for the same energy and voltage, we 
can take account of this factor of increase by writing 

w 

PDmy (7) 

P = 

E cos <A 

with analogous expressions in the case of the previous formulae. 
For aluminium wire, insert the factor 2 in the denominators 
of the weight formulae. 

Another convenient empirical formula for the total weight 
of copper in a three-phase circuit is the following 

M^Kw 
W= 300,000,000 — — -. 
a V^ 

In which M is the distance of transmission in miles, K w the 
kilowatts of energy transmitted at voltage V , and a is the 



THE LINE. 511 

percentage of loss expressed as a whole number. This for- 
mula gives results a few per cent larger than (5) and like it, 
can be made to take account of lagging current so far as ohmic 
drop is concerned by putting cos <f> in the denominator. For 
aluminium wire, put 2 in the denominator as in the other 
formulae. 

It frequently happens in using these formulae that the 
wire indicated by the assumed data, falls between two of the 
ordinary sizes. In large work one can have wire or cable 
made very nearly to the desired size without increased expense, 
but ordinarily one chooses the nearest standard size, prefer- 
ably the next larger. 

So far then as computing the copper required for trans- 
mitting the energy goes, it is a very simple matter to figure 
a transmission line. But inductance is another matter. 
The simplest way of treating it is to deal with it as an addi- 
tional resistance, causing no increased loss of energy for the 
same current, but demanding increased E. M. F. at the gener- 
ator, and affecting consequently the regulation. The resis- 
tance of the line determines the energy loss, the impedance 
the limits within which the impressed E. M. F. must be regu- 
lable. 

For any system of given size and distance of wires worked 
at a given frequency, the inductance like the resistance in- 
creases directly with the length of circuit so that the ratio 
between them is constant, and one can express the impedance 
in terms of resistance by multiplying the resistance by the 
proper impedance factor, when once this ratio is ascertained. 
The same factor converts the ohmic drop into the impedance 
drop which is the quantity here sought. 

In a circuit with wires spaced 8 inches between centres, 
and each r inches in radius, the self-induction in henrys per 
mile is 

L = 0.000322 [2.303 log- + .25] 

r 

which results from translation of the C. G. S. formula 



512 



ELECTRIC TRANSMISSION OF POWER. 



into English measure and common logarithms. From this 
as a basis and from the known resistances, is constructed the 
curves of Fig. 258. They give graphically the impedance 
factors at 60^^ for wires from No. 0000 to No. 4 string 24, 48, 
and 72 inches between centres. 

The impedance factor increases with the size of wire at any 

8.0 



.5 
.4 
.3 

h 

2.0 

< 
o 

lu .9 



2t: 



(HI 

WIRE GAUGE 



diameter 
Fig. 258. 



given spacing because the resistance decreases in proportion 
to the area, and the length of the circuit is not concerned 
since both resistance and inductance increase directly with 
the length so that they remain proportional. For the most 
part the value of the factor ranges from 1.5 to 2.5 so that 



THE LINE. 513 

considering only this matter, the total drop is seldom much 
above twice the ohmic drop. To compensate for the inductive 
drop, then, the generator must have a margin of voltage 
correspondingly greater than that required by the ohmic loss 
alone. 

It cannot be too strongly impressed upon the reader, how- 
ever, that in actual practice these line constants are greatly 
modified by the character and amount of the translating 
devices at the receiving end of the line. To determine the 
actual drop in the Inie, and the regulation required, one must 
take into account both the line and the load. As a rule the 
inductances and capacities of high voltage apparatus are 
rather large compared with those of lines of moderate length, 
but large or small they modify the regulation, for the final 
impedance of the system is the geometrical sum of its com- 
ponents. 

As a practical matter it is the constant effort of the engi- 
neer to keep the power factor of the transmission circuit high, 
so as to avoid the loss due to generating and transmitting a 
large useless component of the current chargeable to lag. 
In working at a bad power factor, not only does the impedance 
ratio rise, but the resistance drop increases for the same energy, 
so that the regulation quickly goes from bad to worse. 

As a general rule the impedance due to the line and load is 
likely to introduce a total line drop two to three times the ohmic 
drop for the same line current, unless helped out by capacity. 
If then the full load drop due to resistance be 10 per cent, 
one must be prepared at the station to furnish 10 to 20 per 
cent extra voltage to compensate for inductive drop. It is 
therefore especially desirable to obtain a high power factor 
at and near full load, to avoid using generators of abnormal 
capacity. The light load power factors cause little trouble. 
The fundamental requirement is that the station should be 
able to hold uniform voltage at the receiving end of the line 
under all circumstances of load. To give good commercial 
results the service voltage should be kept within 2 per cent 
of normal if lighting by incandescents is important, and within 
4 or 5 per cent for satisfactory motor service. 

This means that the conditions of regulation must be thor- 



514 



ELECTRIC TRANSMISSION OF POWER. 



oughly investigated. At any state of load the regulation is 
determined by the vector sum of the impedances in circuit, 
which for regulation at the receiving substation means sum- 



X)0390 



.00350 



» .00330 



; .00320 



i .00310 



.00270 



:::::t:: 








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._ 


















Ah^ 




p - 






-- 






— ? 
































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































^ 




















































































































































































































































































__ 







._ 










I 











0000 000 



1 

GAUGE (B & S) 

Fig. 259. 



ming the impedances of the line and of the receiving circuit 
under various conditions of load. 

First in order comes the actual inductance of the line wires. 
Here as elsewhere in this discussion, the line is assumed to be 



THE LINE. 515 

three-phase with the wires symmetrically arranged at the 
corners of an equilateral triangle. The formula for the co- 
efficient of self-induction, L, which depends entirely on the 
dimensions of the system, has been given, but for convenience 
the values per mile of complete circuit for wires from No. 0000 
to No. 4, strung 24, 48, and 72 inches apart are shown graphi- 
cally in Fig. 259. To reduce to self-induction per wire divide 
by V3. Multiply by 2 7rn to obtain the inductance in ohms 
from the values of L given by the curves. For 60^, 2 tt n = 
377. 

The curves show that for the wires in common use in trans- 
mission work, L does not vary over a wide range, being com- 
monly 3 to 3.5 milli-henrys per mile of circuit. 

There is another cause of increased drop of voltage in alter- 
nating current circuits quite apart from ordinary inductance. 
Some years ago Lord Kelvin pointed out that in the case of 
alternating and other impulsive currents the ohmic resistance 
of conductors is slightly increased. This is for the reason 
that in such cases the current density ceases to be uniform 
throughout the cross section of the conductor. The instan- 
taneous propagation of any current is primarily along the sur- 
face of the conductor, and only after a measurable, though 
short, time is the condition of steady flow reached. 

When the current rapidly alternates in direction the interior 
of the conductor is thus comparatively unutilized, for before 
the flow has settled into uniformity its direction is changed, 
and the original surface flow is resumed. The larger the wire 
and the greater the frequency the more marked this effect. 
Fortunately, with the common sizes of wire and the frequencies 
ordinarily employed for power transmission work, it is quite 
negligible. At 60 periods the increase of resistance due to 
this cause, in a conductor even half an inch in diameter, is less 
than one-half of 1 per cent. Any line wire that is allowable 
on the score of its impedance factor will be unobjectionable 
on this account as well. Only occasionally, as in bus bars for 
low voltage switchboards, is it worth considering, and in such 
cases the use of flat bars, half an inch or less thick, or tubular 
conductors, will obviate the difficulty. 

In computing the sum of the impedances, it is sometimes 



516 ELECTRIC TRANSMISSION OF POWER. 

convenient to include with the Hne the impedances of raising 
and reducing transformers reduced to terms of the full line 
pressure, the primary resistance being increased by the secon- 
dary resistance multiplied by the square of the transformation 
ratio to form the equivalent total resistance, usually not far 
from twice the primary resistance; and the inductance being 
determined from the inductive drop when loaded. 

At the receiving secondary terminals, the measured angle 
of lag due to the load at once tells the story of the relation 
between the power and the idle component of the current. 
For the purpose of determining regulation the items of the 
load need not be considered, if we know the lag angle which 
determines the current components which have been fur- 
nished over the line. For short lines overhead, the line impe- 
dance and the lag angle determine the regulation, but on very 
long overhead lines, and in underground cables, capacity 
plays an important part. 

The capacity of overhead circuits like the self-induction is 
determined by the dimensions of the system, except as there 
may be localized capacity. For the customary three-phase 
overhead circuits the situation has been simplified by Perrine 
and Baum,* who showed that for such circuits the capacity 
acted as if concentrated in three condensers at the middle of 
the line, and star connected to a common neutral point. Upon 
this hypothesis, which leads to sufficiently precise results for 
all cases now practical, the capacity C in microfarads reckoned 
between one wire and netural point for wires r inches in radius 
and spaced d inches apart becomes, per mile, 

.0776 
o — 



2Iog.Q 

Fig. 260 shows graphically the values of C for wires of the usual 
sizes spaced respectively 24, 48, and 72 inches between centres. 
Here again there is a considerable degree of uniformity, C 
ranging ordinarily between .014 and .018. 

The corresponding current equivalent, or charging current, 
* Trans. A. I. E. E. May, 1900. 



THE LINE. 



517 



i, depends like the inductance on the frequency, but also^ 
unHke the inductance, upon the voltage; and in general 

i = p C nV. 
Here, for one wire of a three-phase line, p has the value 

V = .000,003,627. 
For a 60 cycle line, multiply the values from the curve by 



.019 



.018 



.016 



.015 



.014 



.013 



I MM I !'[' iTTi'i' ' 1 r T ' ■ ■ ■ 


























1 : , ! : — -^1 — r-r-| 1 ITT \ 


|i ! } [ : i h -h ;<' — T---- 
























































■ Ml IMj lU.,,,. ■■^,,1: .::_ 








1 , ■ ' : ■ 1--^ 1 ; 1 ' 1 1 i 1 1 1 -- ' 






































1 ! 1 ! 1 1 1 ] t 1 ■ ■; : 



1 00 

wire gauge (b 4. s) 
Fig. 260. 



0000 



2.18 for 10,000 volts line pressure and proportionately more 
for higher pressures. 

As for our purpose the capacity is taken as if localized at 
the centre of the line, ^ must be regarded as flowing through 
one-half the line impedance. If there is localized capacity 
elsewhere, as in case of cables, its charging current, determined 
from the capacity of the cable as above, must be taken as flow- 
ing over the actual length up to the capacity and forms a 
geometrical addition to the capacity just considered. 

We now have in hand the data for figuring the terminal 
voltage of a transmission line from the impressed voltage by 



518 ELECTRIC TRANSMISSION OF POWER. 

summing the several impedances, and computing their resultant 
in view of the current and energy values disclosed by the 
lag (or lead) angle <^ at the terminus. 

For the present purpose the most simple and elegant method 
of making this summation is that of Perrine and Baum (loc. 
cit) * which takes as a starting point the receiver voltage 
which is to be held steady, and treats the power component 
and the idle component of the receiver current as if they 
flowed independently through all the impedances up to the 
receiver as, in effect, they do. 

Let us start then with the receiver voltage and lay out 
Oa, Fig. 261, equal to this voltage on any suitable scale. Here 
the receiver voltage is taken at 10,000. The ohmic drop at 
full non-inductive load we will take as 2,000 volts which lay 
off as an extension of Oa to h. The total current in the system 
is composed of the true energy current, the idle current, and 
the charging current, if any, each of which consumes voltage 
in being forced through the line impedance. Taking them 
up successively, the energy current is / cos <^, <^ being the 
angle of lag or lead at the receiver, and since we are here 
considering full load energy 

ab = I R cos <^ 

i.e., the ohmic drop of the energy current. Now proceed to 
form the ordinary impedance triangle a 6 c as follows. From 
b erect a perpendicular such that 

b c = I (L ci) cos <A 

on the same scale as a b, L^ being the inductance in ohms. 
This can be done by computing the actual inductance from 
the data assumed. Then a c, on the working scale, gives in 
magnitude and direction the total volts consumed over the 
line by the energy-current. If transformers or other apparatus 
are included in this estimate for the line, this fundamental 
triangle a b c must be built of its components geometrically 
as shown in Fig. 56. If the line only is concerned, the point 

* See also Baum, Elec. World & Eng., May 18, 1901, and Trans. Int. 
Elec. Cong., 1904, Vol. II, p. 243. 



THE LINE. 



519 




Fig. 261. 



520 ELECTRIC TRANSMISSION OF POWER. 

c is at once located by striking from a as a centre an arc with 
a radius equal to the impedance factor on the scale of a h, and 
erecting a perpendicular from h to meet it. 

The next step is to determine the magnitude and direction 
of the pressure consumed over the line by the idle component 
of the current. This is at right angles to a c, hence draw a 
line perpendicular to a c, as ^ c d. Then lay off the angle <A 
from a as a centre to the right if lagging, to the left if leading. 
The intercept c c? is the pressure required; for dropping the 
perpendicular d e upon c 6, the triangles a h c and c d e are 
similar, with their corresponding sides by construction respec- 
tively proportional to cos </> and sin <^. Thus 

c e = I R sin <i> 

ed = I (LJ sin^ 

cd = I sin <t> \Ir2 ^ (^ij2 

Now draw Od which is the geometrical sum of Oa, a c, c d and 
we have E, the impressed E. M. F. necessary to give 10,000 
volts at the receiver under the assumed conditions. With E 
as radius, draw the arc d f and E is at once seen to be 14,200 
volts. The point d corresponds to cos <^ = .90. For other 
values lay off the appropriate angles and treat as before. 
For angles of lead lay off the angles on the other side of a c, 
as a g for cos ^ = .90. This gives E2 which thrown down 
upon the voltage axis gives 11,200 volts at the point h. This 
shows less than the normal drop, since a leading current at 
the load can only exist concurrently with condenser effects. 
And capacity in the line remains to be considered. From 

d lay off k = — and k j -= —, when / d becomes the 

impedance for the charging current and j the new impressed 
E. M. F. 

If capacity is an important item, it is easier, since it is 
constant for all values of the load, to lay out d k and k j at 
the start, making d coincide with a and then starting the 
fundamental power triangle from the new position of j as in 
Fig. 262. 

As a matter of fact, line capacity is not an important factor 



THE LINE. 



521 



in transmission, save in rather long lines at high voltage. 
For example, in a 20-mile line at 20,000 volts, of No. 00 wire 
spaced 24 inches, the charging current is about 1.6 amperes 
per wire, the impedance factor of the line nearly 1.7, the 
resistance of half a wire a little over 4 ohms, and the resulting 
E. M. F. for capacity impedance only some ten or a dozen 
volts. 

For partial load regulation note that ah c, Fig. 261, holds 
its shape for all loads and merely changes in magnitude. For 
half load therefore, go half-way up along a c to the point I, 
which corresponds to c of the full-load diagram. Draw the 
perpendicular corresponding to c c? through I. Then for any 
power factor, as .8, the intersection m gives the end of the 
corresponding impressed E. M. F. as before. A system of 




Fig. 262. 



lines parallel with I m and for every tenth of a c, intersecting 
all the power factor lines, makes it easy to determine the 
regulation for almost any sort of load. 

The rise of E. M. F. at the end of a line containing capacity 
is one of the most striking features of alternating current 
working, and while the constructions just given show its 
amount, they do not at first sight disclose its physical signifi- 
cance. The fact is, however, that a condenser is a device for 
storing electrical energy, which is returned to the line in such 
wise that its voltage is added (geometrically of course) to the 
line voltage. It simply amounts to an electrostatic booster 
of enormous efficiency, close upon 100 per cent, taking energy 
from the line and utilizing it in raising the voltage. If the 
capacity is distributed along the line, it takes a very long 
line to do much boosting. If it is concentrated and consider- 
able, as in a cable, the effect may be very striking. 



522 



ELECTRIC TRANSMISSION OF POWER. 



An over-excited synchronous motor, as we have already 
seen, can be made to act Hke a condenser in the system, although 





1 


'able of 


Natural Tangents, Sines and Cosines. 




0) 








Diflf. 


6 








Ditf, 


"^ 


Tan. 


Sin. 


Cos. 


Sin. 


% 


Tan. 


Sin. 


Cos. 


Sin. 


< 








W 


^ 








10' 


0° 


.0000 


.0000 


1.0000 










1 


.0175 


.0175 


.9998 


29 


46° 


11.0355 


1.7193 


.6947 


20 


2 


.0349 


.0349 


.9994 


29 


47 


1.0724 


.7314 


.6820 


20 


3 


.0524 


.0523 


.9986 


29 


48 


1.1106 


.7431 


.6691 


20 


4 


.0699 


.0698 


.9976 


29 


49 


1.1504 


.7547 


.6561 


19 


5 


.0875 


.0872 


.9962 


29 


50 


1.1918 


.7660 


.6428 


19 


6 


.1051 


.1045 


.9915 


29 


51 


1.2349 


.7771 


.6293 


18 


7 


.1228 


.1219 


.9925 


29 


52 


1.2799 


.7880 


.6157 


18 


8 


.1405 


.1392 


.9903 


29 


53 


1.3270 


.7986 


.6018 


18 


9 


.1584 


.1564 


.9877 


29 


54 


1.3764 


.8090 


.5878 


17 


10 


.1763 


.1736 


.9848 


29 


55 


1.4281 


.8192 


.5736 


17 


11 


.1944 


.1908 


.9816 


29 


56 


1.4826 


.8290 


.5592 


17 


12 


.2126 


.2079 


.9781 


28 


57 


1.5399 


.8387 


.5446 


16 


13 


.2309 


.2250 


.9744 


28 


58 


1.6003 


.8480 


.5299 


16 


14 


.2493 


.2419 


.9703 


28 


59 


1.6643 


.8572 


.5150 


15 


15 


.2679 


.2588 


.9659 


28 


60 


1.7321 


.8660 


.5000 


15 


16 


.2867 


.2756 


.9613 


28 


61 


1.8040 


.8746 


.4848 


14 


17 


.3057 


.2924 


.9563 


28 


62 


1.8807 


.8829 


.4695 


14 


18 


.3249 


.3090 


.9511 


28 


63 


1.9626 


.8910 


.4540 


13 


19 


.3443 


.3256 


.9455 


27 


64 


2.0503 


.8988 


.4384 


13 


20 


.3640 


.3420 


.9397 


27 


65 


2.1445 


.9063 


.4226 


12 


21 


.3839 


.3584 


.9336 


27 


66 


2.2460 


.9135 


.4067 


12 


22 


.4040 


.3746 


.9272 


27 


67 


2.3559 


.9205 


.3907 


12 


23 


.4245 


.3907 


.9205 


27 


68 


2.4751 


.9272 


.3746 


11 


24 


.4452 


.4067 


.9135 


27 


69 


2.6051 


.9336 


.3584 


11 


25 


.4663 


.4226 


.9063 


26 


70 


2.7475 


.9397 


.3420 


10 


26 


.4877 


.4384 


.8988 


26 


71 


2.9042 


.9455 


.3256 


10 


27 


.5095 


.4540 


.8910 


26 


72 


3.0777 


.9511 


.3090 


9 


28 


.5317 


.4695 


.8829 


26 


73 


3.2709 


.9563 


.2924 


9 


29 


.5543 


.4848 


.8746 


25 


74 


3.4874 


.9613 


.2756 


8 


30 


.5774 


.5000 


.8660 


25 


75 


3.7321 


.9659 


.2588 


8 


31 


.6000 


.5150 


.8572 


25 


76 


4.0108 


.9703 


.2419 


7 


32 


.6249 


.5299 


.8480 


25 


77 


4.3315 


.9744 


.2250 


7 


33 


.6494 


.5446 


.8387 


24 


78 


4.7046 


.9781 


.2079 


6 


34 


.6745 


.5592 


.8290 


24 


79 


5.1446 


.9816 


.1908 


6 


35 


.7002 


.5736 


.8192 


24 


80 


5.6713 


.9848 


.1736 


5 


36 


.7265 


.5878 


.8090 


24 


81 


6.3138 


.9877 


.1564 


5 


37 


.7536 


.6018 


.7986 


23 


82 


7.1154 


.9903 


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4 


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23 


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8.1443 


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22 


84 


9.5144 


.9945 


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3 


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.8391 


.6428 


.7660 


22 


85 


11.430 


.9962 


.0872 


2 


41 


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.6561 


.7547 


22 


86 


14.300 


.9976 


.0698 


2 


42 


.9004 


.6691 


.7431 


21 


87 


19.081 


.9986 


.0523 


1 


43 


.9325 


.6820 


.7314 


21 


88 


28.636 


.9994 


.0349 


1 


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.9657 


.6947 


.7193 


21 


89 


57.290 


.9998 


.0175 


0.5 


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1 .0000 


.7071 


.7071 


20 


90 


OC 


1.0000 


.0000 





THE LINE. 523 

at less efficiency, and the angle of lead which it must give to 
the receiver current in order to produce any desired effect 
on the voltage can be deduced from the construction of Fig. 
261. Practically, therefore, synchronous machines are very 
valuable adjuncts in regulation. If rotary converters are 
used, as in handling a railway load, they can be so compound 
wound from the direct current side as to compensate for the 
effect of their own changing load upon the receiver voltage. 
The same thing can be even more easily done with a motor 
generator. Now and then on very long high voltage lines 
it is desirable to add inductance at light loads to preserve the 
regulation.' For if one lays out the light load conditions in 
Fig. 261 the capacity triangle, dk j becomes relatively im- 
portant. For convenience in computations a table of natural 
sines, cosines, and tangents is annexed. The column of dif- 
ferences for the sines holds good for cosines of the same numer- 
ical values. 

Really the most serious practical difficulties in an ordinary 
alternating plant are those in which the generator is involved 
by inductances in the system. These are often of far greater 
moment than the impedance factor of the line. An inductance 
in the system produces two effects on the generator — first, 
as just noted, it demands a larger current to deliver the same 
energy; second, it tends to beat down the E. M. F. of the 
machine. This effect is analogous to that produced by shift- 
ing the brushes of a continuous current generator away from 
the position of maximum E. M. F. (See Chapter V.) 

This reaction of the armature is serious in that it not only 
demands a considerable increase in the exciting current, but 
causes a severe strain on the insulation when it suddenly 
ceases. It is not uncommon to find an alternator that requires 
on a heavy inductive load double the light-load excitation of 
the field. For instance, if the voltage be 2,000 on open cir- 
cuit, the excitation may have to be increased on inductive 
load to a point that on open circuit would give 4,000 volts. 
If, now, this load is cut off, or the line is broken, the insula- 
tion will be exposed, momentarily, at least, to double the 
normal voltage. 

Such generators should not be used on inductive loads or 



624 ELECTRIC TRANSMISSION OF POWER. 

in any case where the extra strain on the insulation is impor- 
tant. It is perfectly easy to build a generator which requires 
only 10 to 15 per cent more excitation at full and inductive 
load than at no load, and such machines should be used in all 
cases where a steady voltage under all working conditions is 
needed. The other type has its uses, but the general work of 
power transmission is not one of them. With a properly 
designed machine, inductive load is little to be feared. 

Another possible source of danger is that under certain 
conditions of inductive load, the reaction of the load on the 
generator, without materially lowering its effective voltage, 
may so change the shape of the E. M. F. wave as to give to it 
an abnormally high maximum, and thereby greatly to increase 
the strain on the insulation. This effect may readily occur, 
but usually in so small a degree as to be of little moment. 
Occasionally, owing to a combination of severe inductive load 
and badly designed generator, the results may be somewhat 
formidable, the more so as the change takes place under 
heavy load and not, as in the case just treated, only on open 
circuit or a sudden light load. The rise in pressure thus pro- 
duced may amomit to several times the nominal voltage. The 
same sound principles of design that insure good regulation 
under changes of load will obviate any danger of this kind. 
In fact, most of the possible disturbing factors in alternating 
current work become negligible in an installation carried out 
with regard for the general principles of good engineering. 

These abnormalities of voltage lead naturally to the con- 
sideration of another far more serious, due to the static capac- 
ity of the system. Of course, the fact that under certain 
circumstances capacity in the system will cause a lessening 
of the apparent ''drop" on the line, or even overcome it alto- 
gether and show a higher voltage at the receiving end than 
at the generator, is already well known to the reader. Under 
certain conditions, however, this rise may become cumula- 
tive, producing electrical resonance, the fundamental prin- 
ciples of which have already been described. 

Every electrical system has as we have already seen, a 
definite period of oscillation determined by its particular prop- 
erties. If we could apply an instantaneous electromotive 



THE LINE. 525 

stress to any point of it, the effect would be that the result- 
ing strain would travel back and forth with a definite fre- 
quency until its energy would be completely exhausted by 
doing work on various parts of the system. The action 
resembles that which takes place when we strike the end of 
a long rod with a hammer. An impulse is sent out at a rate 
depending on elasticity, density, and so forth, travels to the 
end of the rod, is reflected, and so goes on swinging back and 
forth until the energy is frittered away. This corresponds 
to electric oscillations on open circuit. 

The two properties of an electrical system which determine 
its vibration period are its self-induction, which is analogous 
to inertia, and its capacity, which resembles elasticity in the 
dielectric, capable of taking up and returning energy. Resis- 
tance, like intermolecular friction in the rod just referred to, 
determines the rate at which the vibrations will die out by 
yielding up their energy to the system, but has ordinarily 
a negligible effect on the vibration period. 

This period in an electric circuit is given by the formula: 

T = .00629 Vl^= ^ -^LC. 

In this T is the natural time period of the circuit expressed 
in seconds, L is the coefficient of self-induction in henrys, 
and C the capacity in microfarads. For example, suppose we 
are dealing with a circuit of which the capacity is two micro- 



E 



■JfifJ^if^pfJfjnfJp TmIcBOFABABS 



«£NEfiATOa 



Fig. 263. 

farads and the self-induction one henry. Let it be arranged 
as in Fig. 263. For simplicity the inductance and capacity are 
shown localized and in series as would happen if a line ran 
through a group of series transformers and thence into a cable. 
If the line were open-circuited beyond the cable, we might find 
a very severe strain on the cable insulation. The period of 
this line would be .00887 second — about 113 cycles per second. 
If this should chance to be the frequency of the generator it 



526 ELECTRIC TRANSMISSION OF POWER. 

would be in resonance with the Hne, and each wave of E. M. F. 
sent out by the generator would add itself to another wave just 
starting out in the same direction. A period later these two 
added E. M. F.'s would be reinforced by the next generator 
wave, and so on indefinitely. 

The only thing which prevents the resultant voltage from 
rising indefinitely is the effect of energy losses in causing each 
wave to die out gradually as it continues its oscillations, so that 
only a limited number of waves can add materially to the 
resultant E. M. F. across the terminals of the capacity. 

In a given circuit the relation between the initial voltage 
and the voltage of resonance can be easily determined to 
a fair degree of approximation. It is, neglecting minor reac- 
tions, as we have already seen, 

^. ^ ^ ^ 

E' = E. 

R 

In this equation E' is the rise of E. M. F. due to resonance, 
n the frequency, L the self-induction in henrys, R the ohmic 
resistance, and E the initial voltage. Applying this formula 
to the case just discussed, and assuming the resistance of the 
line to be 15 ohms and the initial voltage to be 2,000, we find 

^, 113 X 1 X 2,000 ,rr.n.^. 

E^ = = 15,066 volts. A very moderate Ime 

15 ^ 

voltage might thus, in a resonant line, give rise to a pressure 
quite capable of rupturing any ordinary cable, or causing serious 
trouble on an overhead line, to say nothing of greatly increas- 
ing the danger to persons and property. If the working 
pressure were 10,000 or 15,000 volts, the E. M. F. of resonance 
might theoretically rise to an appalling amount. 

Fortunately the theoretical value is in practice much reduced 
by hysteretic losses and Foucault currents in any iron-cored 
coils in circuit, waste of energy in the dielectric, and other 
minor causes of damping the electrical oscillations, even when 
resonance is complete. Still, dangerous rises in voltage are 
very possible. When the frequency of the applied E. M. F. 
differs somewhat from the natural period of the line, resonant 
effects can evidently still take place, but in a rapidly lessen- 



THE LINE. 527 

ing degree; when the oscillations are strongly damped by the 
presence of iron, the total resonant rise is considerably dimin- 
ished, but it varies less rapidly as the resonant frequency is 
departed from. 

A resonance curve for various capacities shows that the 
rise of voltage extends over quite a wide range of variation 
of capacity, but is large over but a small range. The shape 
of such a curve necessarily varies widely, as the resonance is 
more or less damped by resistance, iron-cored coils and so 
forth; but we may be quite sure that the maximum resonance 
will occur at not far from the point indicated by our equa- 
tion for the vibration period of the circuit, and that the maxi- 
mum E. M. F. of resonance will usually be considerably less 
than that given by the theoretical equation. 

In practical alternating circuits the current wave is never 
truly sinusoidal, but consists of a main or fundamental wave 
with the odd {i.e., 3d, 5th, 7th, etc.) harmonics of various 
amplitudes superimposed upon it. In nearly every case the 
third harmonic is the most prominent and is quite capable of 
causing resonance, even to a dangerous degree, if it happens to 
fall in with the frequency of the system. The point at which 
resonance occurs and the rise of E. M. F. are found for the 
harmonics by the formulae already given. 

So far as the line is concerned, the facts regarding resonance 
can be easily computed with tolerable accuracy. From well- 
established data it is evident that the line capacities and 
inductances are generally so small as to make the oscillation 
period so short as not to correspond with the frequencies in 
ordinary use except in the upper harmonics, which are generally 
of small moment, although one case of severe resonance from 
a higher harmonic (probably the 7th) has come to the author's 
notice. For example, with a 7th harmonic of 1,000 volts 
amplitude on a 10,000 volt line at 60^, having an inductance 
of .2 henry and a resistance of 20 w, the rise due to resonance 
might be some 40 per cent of the line voltage. 

It must be remembered that not only the line capacity, but 
the capacity of the sending and receiving apparatus, must be 
considered. The former is but small, except in the case of 
underground or submarine cables, for which the capacities are 



528 ELECTRIC TRANSMISSION OF POWER. 

likely to be from i to ^ microfarad per mile, as ordinarily 
manufactured. High- voltage devices, like synchronous motors, 
generators, and transformers, often may have static capacities 
of several tenths of a microfarad, and inductances of several 
hundredths of a henry. Resonance may involve the whole 
system, or may at times be started in a minor degree in some 
branch in which the natural oscillation period happens to be 
just right. 

As a matter of fact, experience seems to show that one is 
not likely to stumble upon very serious resonance in overhead 
lines, although in cables it is easily possible. On the other 
hand, it is more than likely that resonance of a minor kind, 
mostly from harmonics, is far commoner than is generally sup- 
posed. It will be noted from the data given that L and C 
on simple overhead lines do not vary over a wide range in dif- 
ferent sizes of wire at the ordinary spacings. Both increase 
directly with the length of the line, and so of course does ^LC 
on which the natural frequency of the line depends. Bearing 
this in mind, one can get a roughly approximate idea of the 
natural frequency on which resonance depends. For fairly 
long lines, say between 50 and 100 miles, N, the frequency in 
question is likely to fall between 300^ and 500^, being 
proportionately less for longer lines and greater for shorter 
lines. 

Obviously this value makes resonance with the funda- 
mental generally out of the question, but gives a good chance 
for the 5th and 7th harmonics. 

Pure resonance with a periodic E. M. F. due to the generator 
is therefore practically confined to harmonics, but there are 
other sources of abnormal pressure on a transmission line. 

Chief among these is surging, which is due to the oscillations 
of energy when a circuit which contains inductance and capa- 
city is broken. It is a resonant phenomenon, depending as 
it does on the line period, but ordinarily it falls in with no 
source of cumulative impulses, which separates it from reso- 
nance, ordinarily so called. 

The theory of surging is comparatively simple. When a 
circuit containing inductance and capacity is broken when 
carrying a current /, a certain amount of energy is left momen- 



THE LINE. 529 

tarily stored in the form of the electro-magnetic stresses in the 
system. The energy thus cut off in transit, as it were, is 

LP 

This, for lack of other outlet, is thrown into the capacity, and 
then, thrown back by it spring-wise, goes on thus oscillating 
with decreasing amplitude until it is frittered away by ohmic 
and other sources of loss. 

But the energy stored in a condenser is 



where E^ is the voltage across its terminals. And since in 
surging, the energy in the condenser is that received from the 
electro-magnetic storage in the line, 

LP _ E^C 
~2~ ~"2~* 

C is here taken in farads. The frequency of the oscillation is 
evidently that naturally belonging to the system. Now this 
frequency involves a relation between L and C, being 2"^ N = 

— =^ ; and now solving the energy equation just given for E^, 
-^LC 

the E. M. F. of the surge, one obtains two correlated expres- 
sions for E, one involving L and the other C, and both in terms 
of the frequency and current, as follows: 

E, = 27rN LI, (1) 

E, = (2) 

' 27rNC ^ ^ 

Knowing / the current broken, L and C, the value of E is 
obtained at once by substitution in either above equation (1) 
or (2). 

The Ej thus obtained is the alternating voltage as ordinarily 
reckoned. Its crest is approximately E^ V2 volts, more if 
the wave be peaked, and the maximum strain tending to 



530 ELECTRIC TRANSMISSION OF POWER. 

break down insulation is this plus the crest of the impressed 
E. M. F. It is not uncommon to find waves sufficiently peaked 
to give E'max = l.QE. 

Based on somewhat rough approximations to the average line 
constants, Baum has given an approximate equation E^ = 200 / 
which is sufficient to give a working relation between the 
current in amperes broken and the voltage rise of the surge. 
This of course does not hold with cables in circuit or when the 
inductance and capacity of apparatus are taken into account. 

In any case there is a good chance of opening the circuit 
at some other instant than that of maximum current. When 
ordinary switching is going on, especially with oil switches, 
there is rarely much surging, but a short circuit, particularly 
in a line containing cables, is likely to make mischief. 

Still apart from surging, is the group of impulsive disturbances 
loosely classified as '^static." They are exceedingly common, 
since they result from all sorts of sudden changes of load, 
switching on feeders, cutting in transformers, and so forth. 

Suppose, for example, a long line is thrown on. There is 
a sudden rush of current sending an impulse along the line. 
This wave may be very abrupt, and, at the end of an open 
line or at any electrical obstacle like inductance or a sudden 
reduction in capacity, is wholly (for open circuit) or partially 
reflected, and as the phase changes suddenly during reflec- 
tion there is an impulsive rise in pressure, up to double the 
wave voltage for total reflection with its phase change of a 
quarter cycle. 

The reflected wave in running back may coincide with the 
crest of a secondary disturbance, or in very extreme cases 
may fall into resonance; but as a general rule, the effect is 
merely a sharp rise of pressure at the reflecting point, amount- 
ing to an increase of perhaps 50 to 100 per cent in the nominal 
pressure. In one particular the results may be serious, for 
the wave front in thus charging a line may be so abrupt as 
to be equivalent with respect to self-induction to a current 
of enormous frequency. Reaching an obstacle like the pri- 
mary of a high-tension transformer, the full crest of the wave is 
upon it before the front has had time to penetrate far into 
the coil, and there may thus result a dangerous concentration, , 



THE LINE, 631 

of potential in the outer layers of the coils, sufficient to cause 
punctures of the insulation. Grounds, short circuits, induced 
or direct lightning discharges, or any sudden and violent 
change of potential from any cause, may start a potential 
wave abrupt enough to produce breaking down of insulation. 
In practice ''static" comes thus from a wide variety of causes, 
and, being impulsive, seldom is so much a source of danger 
as a heavy surge or true resonance. Yet it sometimes pro- 
duces punctures that are followed by the line current with 
serious results. A very good account of " static " may be 
found in two papers by Thomas.* In point of fact, resonance, 
surging, and static may cooperate in the same phenomenon, 
and it is generally difficult to analyze the result on the avail- 
able evidence. The moral of all this is that, in the insulation 
of high-voltage apparatus and lines, a considerable factor of 
safety must be allowed, since the insulation may be subjected 
to strains considerably greater than those due to the rated 
voltage. Probably the most dangerous condition is a surge 
following the breaking of a short circuit. With the relations 
existing on overhead lines, between L C and R one is not 
likely to get more than 3 to 4 times normal voltage. It is 
well to estimate the surge for a short circuit midway the 
line, and use the factor of safety thus indicated, bearing in 
mind, as a favoring factor, the fact that the arc from a short 
circuit softens the suddenness of the break, and lets down the 
current. The worst cases will be met on underground systems, 
and it is worth noticing that for a given amount of energy 
transmitted the higher the voltage the less the current, and 
the less the voltage rise due to interrupting that current. 
On the other hand, near the highest voltages now in use there 
is a tendency to trench on the factors of safety in insulation. 
We have now investigated all the important factors that 
enter into the design of a transmission line, whether for direct 
or alternating currents. Let us review them with the idea of 
seeing how they enter into practical cases. First comes the 
all-important question of initial voltage, involving the choice 
between the direct generation of the working pressure or its 
derivation from transformers, if alternating currents are used. 
* Trans. A. I. E. E., March, 1902, and June, 1905. 



532 ELECTRIC TRANSMISSION OF POWER. 

We have already seen the practical limitations of voltage for 
direct currents. With alternators the commutator troubles 
are absent, and the limitations are those imposed by generator 
design. The higher the voltage of a dynamo, the more space 
on the armature must be allowed for insulation, thereby cut- 
ting down the output of the machine. Hence the practicable 
voltage depends on the size of the generator. 

In a general discussion it is difficult to make exact state- 
ments as to what can or cannot be done, but experience seems 
to show that at present 10,000 to 13,500 volts are the 
greatest pressures that can economically be derived from the 
generator, even in very large units, while in units of 100 or 
200 KW it is seldom advisable to go above 3,000 to 5,000. 
Higher voltage than this has been attempted, but there is good 
reason to believe that, except in very large machines, the loss 
due to increased space required for insulation outweighs the 
possible gains. 

As to loss in the line, much has been said already, and the 
best advice that can be given is to make a few trial computa- 
tions along the general lines indicated. Almost every case 
will require special treatment in certain particulars, depending 
on the conditions of service. For example, a common com- 
plication is the supply of power or light, or both, at a point 
perhaps half-way along the line. Then, according to the 
amount and kind of service, it may be desirable simply to 
tap the line for power and use a motor generator for 
lights, to establish a substation with regulating apparatus, to 
compound the generator for the point in question and use 
either of the above methods at the end of the line, to install 
rotary transformers, or to run a separate line with regulators 
at the generating station. Such details will be treated at 
length later. 

The line structure is generally of bare copper wire carried 
on strong wooden poles. Do not put it underground unless 
you have to do so for reasons now obvious. It may be 
necessary to insulate portions of the wire, but it is best not 
to put much faith in an insulating covering. Instead, it is 
desirable to make a very thorough job of insulation at the 
supports, and provide for the easy inspection of the line. 



THE LINE. 633 

In using alternating currents, inductance in the line must 
always be considered. Practically it means raising the voltage 
of the generator or raising transformers, unless a fair part of 
the load is in synchronous motors which can be employed 
to counteract the inductive drop. In nearly every case its 
real importance is small, in spite of its scaring the uninitiated 
now and then. 

So, too, with the inductive load. Its real effect is merely 
to increase the current in the line by a small amount, usually 
less than 20 per cent, and to demand increase of excitation 
at the dynamo. If this is so designed as to regulate badly, 
an inductive load will render it difficult or impossible to keep 
a uniform voltage. On the other hand, a generator capable 
of holding its voltage from no load to a full and inductive 
load with an increase of only 10 or 15 per cent in the exciting 
current, will usually give no trouble whatever with reasonable 
attention to the regulators. 

The total net result of inductance in line and load is to 
call for a well-designed generator with good inherent regula- 
tion and a reasonable margin of capacity. One who know- 
ingly installs anything else deserves all the troubles that 
inductance can produce. 

Rise in voltage, on throwing off the load or through distor- 
tion of the current wave by an inductive load, can be reduced 
to insignificance by employing a proper generator, as just 
noted. Aside from this, a mixed load, particularly if it con- 
sists in part of synchronous motors, seldom has a bad power 
factor or great and sudden changes in its amount. Exception 
must here be made with respect to the constant current trans- 
former systems exploited of late in connection with series 
alternating arc lamps. These, unless fully loaded, give a 
severely inductive load, and must be thrown upon the circuit 
very carefully to avoid serious fluctuations of voltage. 

As regards static disturbances, few overhead systems have 
capacity enough to give cause for alarm. Difficulties are to 
be looked for chiefly on very long lines, and those composed 
in part of undergroimd or submarine cables. In these cases 
one may sometimes knov/ the conditions well enough to cal- 
culate the actual result in rise of voltage. More often the 



634 ELECTRIC TRANSMISSION OF POWER. 

data are incomplete, and the simplest way out of the diffi- 
culty is to try the effect of varying the capacity of the system 
before it goes into regular operation. If the addition of a 
condenser, say of one-third microfarad, makes a sharp varia- 
tion in the voltage, look out for resonance and investigate the 
capacity of the system, step by step. A change of capacity 
or inductance can be made sufficient to avert any serious 
danger of resonance imder ordinary conditions. Resonance 
chargeable to the variation of harmonics under changes of 
load and to changes in inductance and capacity due to appara- 
tus used on the system, is hard to foresee, and must be treated 
symptomatically when it chances to appear. 

In the practical computation of a line, the question of 
allowable drop is generally settled by the regulation desired. 
Too much loss makes it impossible to give good service, and 
a loss at full load of 10 to 15 per cent in the line and trans- 
formers is about as much as can be endured, save on very long 
lines where one has to make a virtue of necessity. Eight or 
ten per cent loss in the line proper is a common figure unless 
power commands a very high price, or a limited source must 
be fully utiHzed. As to voltage, 2,000 to 3,000 is the max- 
imum which can conveniently be used in a general distribu- 
tion without step-down transformers. Hence many little 
plants sending power only two or three miles use such 
voltage. 

For serious transmission work, nothing less than 10,000 
volts is worth considering. For 10,000 to 14,000 volts excel- 
lent high-voltage generators are available, and save some- 
thing in cost and efficiency. Roughly one can say that the 
use of the high-voltage generator saves about $6 per kilowatt 
transmitted. On going to a higher voltage with transformers 
then one must be able to save $6 per KW in the line out of the 
cost for the line at 10,000 to 12,000 volts, in order to make 
the change worth the while. For any proposed voltage and 
distance one can readily settle the economics of the question. 
The nominal saving of $6 should, however, be verified for the 
machines and equipment considered, since prices of generators 
sometimes vary very irregularly. Let us suppose, for example, 
that we are investigating the advisability of using 12,500 volts 



THE LINE. 636 

from the generator or 25,000 from raising transformers. The 
latter will save 75 per cent of the copper required by the 
former, which for equahty should cost $6. The two schemes 
will then be equal in cost at a distance for which the lower 
voltage demands $8 per KW for copper, the percentage losses 
being taken as the same. Now reduce the total copper cost 
to pounds, insert in equation (5), page 510, and solvefor Z)^, or 
put the total cost in (6) and solve for D,^. 

With 15 cent copper, 1,000 KW, and 10 per cent loss, the 
critical distance is just over 5 miles. With copper at 20 to 25 
cents this distance is still further reduced, and most plants 
demand high voltage. In general, there will be few transmis- 
sions of half a dozen miles in which it will not pay to raise the 
voltage and install transformers. But in any case where one for 
any reason does not wish to go to the neighborhood of 20,000 
volts on the line, the high-voltage generator is preferable. 

In leaving generator voltage, therefore, go to at least 20,000 
volts and preferably to 30,000. Above that there should be 
more caution in examining adverse conditions; but with a 
reasonably good climate and topography, 40,000 to 60,000 
volts are entirely practicable pressures, and in a few years 
we shall probably be working at 80,000 to 100,000. 

Voltage and loss being settled, the next thing is to lay out 
the line conductors, following the copper formulae already 
given. Then with the approximate dimensions found, con- 
struct the regulation diagram, and plan in so far as may be 
the load to aid the regulation. It is seldom that you cannot 
find at least one big synchronous machine, the excitation of 
which can be controlled. On very long lines look out especially 
for the effects of capacity at light loads. Sometimes a few 
large induction motors steadily loaded prove good counter 
irritants. With the load roughly blocked out, look into the 
conditions of resonance, surging, and so forth, and plan the 
insulation precautions, keeping a special eye on cables and 
their junctions to aerial lines. 



CHAPTER XIV. 

LINE CONSTRUCTION. 

The first consideration is the general question of location. 
Other things being equal, it is obvious that a direct line is the 
best, but as a matter of fact it is seldom altogether practicable. 
A line must above all things be secure against interruptions, 
and with this in view, both the location and the constructional 
features should be determined. 

In smooth and easy country, a nearly straight line can usu- 
ally be laid out. For large plants carrying large amounts of 
power at high voltages, it is often desirable to buy the right of 
way outright. Such has mainly been the policy pursued in the 
transmission from Niagara to Buffalo, and, while expensive, it 
gives an absolute command of the situation. In some States 
electric light and power companies are given the right of 
eminent domain to make such ownership possible. 

In cases wherein the purchase of such a location is imprac- 
ticable or would, as often happens, involve very serious expense, 
the best thing is to secure right of way along the public roads, 
so far as they can be conveniently utilized, and right of way 
for the pole line through such private property as may be in 
the contemplated route. Rights along the public roads are 
very desirable, as giving capital facilities for line inspection 
and repair without added expense. It is well, in addition to 
securing rights from the local governing body, to establish 
friendly relations with the abutters and to secure a definite 
understanding as to interference with trees, proximity to build- 
ings, and the like. Right of way merely for the line across 
private lands, with proper facilities for access, can generally 
be cheaply secured. Many owners are public-spirited enough 
to give it for the asking, or for very reasonable compensation, 
when a strip of land has to be taken for a roadway. 

In small transmissions the public roads are most desirable 
as a route, using private lands only for occasional shorts cuts. 

536 



LINE CONSTRUCTION. b^l 

Since a good road along the pole line is highly desirable, 
the route should be taken through clear and accessible country, 
so far as is possible. 

Places to be avoided when possible, even by a detour, are 
marshes, where poles are always hard to set and maintain, and 
roads are difficult to construct; heavily wooded country, 
where there is constant danger to the line from falling 
branches and the like; and rough rocky slopes, where construc- 
tion is difficult, and the line, when constructed, is highly inac- 
cessible. Sometimes the topographical conditions are such 
that these difficulties have to be met, but they are always 
serious. 

In a wooded region the only proper plan is to secure right 
of way broad enough to permit clearing away the trees so 
that they cannot interfere with the line wires, even were 
branches to be blo\vn off in a storm. Nothing short of a hurri- 
cane sufficient to blow down large trees should possibly be able 
to cause trouble; and when the neighboring trees are danger- 
ously high, careful watch should be kept, and any weak or de- 
caying tree at once cut dowTi. The right of way may be some- 
what expensive, but the service must not be liable to inter- 
ruption by so probable a thing as the breaking of a branch. 
It must be remembered that in high- voltage transmissions a 
twig as big as a lead-pencil may, by falling across the line, start 
an arc that will shut down the plant. Sometimes the use of 
extra long poles may enable one to carry the wires clear of pos- 
sible obstructions of this sort. 

In mountainous regions poles may have to be set in very 
bad locations, and sometimes for long stretches every hole 
may have to be blasted at a cost of $5 to $10 per hole, but 
such contingencies are not very common, and may often be 
avoided b}' a moderate detour. It is better to go around a 
mountain than over it, unless the distance is considerably 
greater. When these questions arise they should be answered 
by preliminary estimates. The country should be carefully 
inspected and the relative costs of various routes looked into.. 
For a uniform country the cost of poles and construction is 
directly as the distance, and the cost of copper directly as the 
square of the distance. 



538 ELECTRIC TRANSMISSION OF POWER. 

In case the direct line leads into difficult country — over, for 
example, a rocky hill where the poles would be hard to place 
and much blasting would have to be done — a detour often may 
cheapen construction. A brief computation will give the 
facts. Suppose a 10-mile transmission of about 500 KW at 
10,000 volts, for simplicity assumed to be on the monophase 
system. The line would have to be about No. wire for, say, 6 
per cent loss, and the total weight of copper would be about 
33,000 lbs. Suppose the average cost of poles and insulators 
in position to be $5 in the open country, but that the direct 
route lies for a mile over a rough hill, where holes would have 
to be blasted and poles would be difficult to place. The extra 
cost of this mile might readily be $500 to $600. Now if a devia- 
tion of a mile would clear this hill, it would probably pay to 
abandon the direct route. By taking the shortest available 
course, the actual increase in the length of the route would 
probably not exceed half a mile. This would increase the 
weight of copper for the same loss by about 10 per cent, $495 
at 15c. per lb., and would increase the cost of the pole line by 
about $250 more. In such a case the increased accessibility 
of the line, and the lessened cost of providing a road for inspec- 
tion and repairs, would more than compensate for the small 
difference in expense. 

The same reasoning holds with respect to avoiding other 
obstacles by making detours. It often pays to go somewhat 
out of the way to utilize the public roads, to cross rivers on 
existing bridges, and so forth. A few experiments on the route 
constructed on paper, after careful inspection of the country, 
will usually show the most advantageous line to follow. The 
old and simple process of sticking pins in the map and follow- 
ing up the line with thread is generally the easiest way of getting 
the approximate distances. 

In mountainous country a direct line is often out of the 
question, and the line has to conform to existing trails with 
such shorts cuts as may be possible. An occasional long span 
will sometimes lessen the cost of the line materially. Rivers 
and lakes often form very serious obstacles to line construc- 
tion and call for much skilful engineering. The former can 
often be crossed on existing bridges or by long spans, which 



LINE CONSTRUCTION. 



639 



will be discussed later, but the latter usually have to be gone 
around, although sometimes cables may have to be carried 
under water, or long suspension spans erected carrying the 
conductors clear across. The latter plan is preferable in most 
cases, and a cable should be taken only as a last resort, imless 
in the rare case of the obstacle being near one end of the line, 
so that the cable may be for the originating or the receiving 
voltage. 

Nearly all long lines have to encounter more or less s-erious 
obstacles of the sorts mentioned, and as a rule they cause con- 
siderable deflections from a straight course. Sometimes devia- 
tions are desirable merely as the cheapest way of reaching en 
route localities where power is to be distributed, a matter 
which a few trial computations will settle. 



LINE W^IRE. 

As already mentioned, copper is the best and most usual 
material for conductors; soft-drawn copper under ordinary 
circumstances, hard-drawn when extra strength is desirable. 
No other material gives so advantageous a combination of con- 
ductivity and tensile strength for nearly all purposes. The 
tensile strength of the copper is raised by hard drawing from 
about 34,000 to 35,000 lbs. per square inch to 60,000 or even 
70,000, and the resistance is only raised 2 to 4 per cent, the 
latter amount only in small sizes. Often a medium hard-drawn 











Tensile Strength 


Permissible 


Gauge 


Diameter 


Area Circu- 


Wt., Lbs., per 


(Ultimate) 


Tension 


B.&S. 


Mils. 


lar Mils. 


1,000 Feet. 


Based on 34,000 
Lbs. per Sq. In. 


with Factor of 
Safety 5. 


0000 


460,000 


211,600 


640.73 


5,640 


1,128 


000 


409,640 


167,805 


508.12 


4.480 


896 


00 


364,800 


133,079 


402.97 


3,553 


711 





324,950 


105,592 


319.74 


2,819 


564 


1 


289.300 


83.684 


253.43 


2.235 


447 


2 


257,630 


66,373 


200.88 


1,772 


344 


3 


229,420 


52,633 


159.. 38 


1,405 


281 


4 


204,310 


41,742 


126.40 


1,114 


223 


6 


181,940 


33,102 


100.23 


884 


177 


6 


162,020 


26,250 


79.49 


• 700 


140 


7 


144,280 


20,816 


63.03 


556 


111 


8 


128,490 


16,509 


49.99 


440 


88 



540 ELECTRIC TRANSMISSION OF POWER. 

wire is used having a tensile strength of, say, 45,000 to 50,000 
lbs. per square inch. Such wire is materially stronger than 
the annealed wire, and yet is much easier to handle than such 
hard-drawn wire as is used for trolley wires. 

For line copper the wire should be free from scale, flaws, 
seams, and other mechanical imperfections. It should be very 
close to its nominal gauge, variations of 1 to 2 mils being the 
largest which should be tolerated, and should be within 2 per 
cent or less of standard conductivity, as given for pure copper 
in tables of wire. 

The foregoing table gives the standard mechanical constants 
of the sizes of wire commonly used in power transmission work. 

The various constants should none of them fall short of these 
tabulated values by more than 2 per cent. 

For hard-drawn copper wire the tensile strength should not 
fall short of 1.75 times the values given for annealed wire, save 
in case of wires intentionally drawn only to medium hardness, 
in which case the factor is generally about 1.5. Medium hard- 
drawn copper is strongly to be recommended for transmission 
work, and has to a great extent replaced ordinary soft-drawn 
copper. The elastic limit of hard-drawn copper wire ranges 
from 30,000 to 40,000 lbs. per square inch according to the 
nature of the drawing. 

When in use, wire is subject to serious mechanical strains, 
due in the first place to its weight and normal tension, second 
to variations in tension by change of temperature, and third 
to extraneous loads like ice and wind pressure, separately or 
combined. These last-mentioned strains are sometimes for- 
midable and must be carefully taken into account, particularly 
in cold climates. 

When a wire is suspended freely between supports, it takes 
a curve known technically as the catenary. The exact solution 

__c 



D 

Fig. 264. 



of its properties is. very difficult, but for the case in hand the 
catenary comes very close to the parabola, a much simpler 
curve to compute; and based on this approximation the follow- 



LINE CONSTRUCTION. 541 

ing simple deductions can be made: If a wire be stretched 
between points A and B, Fig. 264, it assumes the curve A D B. 
The thing to be determined is the relation between the length 
A B (which we may call L, the length of span), the vertical 
deflection d, at the middle point of the span, and the tension 
on the wire at A or 5 as a function of its weight. This relation 
is as follows: 



or transposing, 



d=8y. (2) 



Here L is the length of the span in feet, d the central deflec- 
tion in feet, w the weight of the wire in pounds per foot, and 
T the maximum tension on the wire in pounds. 

These equations show that with a given wire the tension 
varies inversely as the deflection for a given span, and that for 
a given tension and wire, the deflection must increase with the 
square of the span. Obviously, shortening the span and in- 
creasing the deflection eases the strain on wire and renders 
the construction more secure, but shortening the span adds 
considerably to the cost, and increasing the deflection increases 
the danger of the wires swinging in the wind and touching each 
other. To prevent this, the deflection should not much 
exceed twice the horizontal distance between wires. 

The application of the formulse can be shown by an ex- 
ample. Suppose we are stringing No. 00 wire on poles 100 ft. 
apart. What is the least deflection allowable with a factor of 
safety of 4? This means that T must not exceed one-fourth 
the breaking strain of the wire, which fraction from the table 
is 888 lbs. The weight per foot from the table is .4 lb. Sub- 
stituting in equation (2) we have: 

(100)2 >^ 4 / 

^ = o ooo = '^^ foot = 6.8 inches. 
8 X 888 

This minimum deflection should not be exceeded in this case, 
and hence must be applicable to the lowest temperature to 
which the line is to be exposed. At whatever temperature 
the wire is strung, enough deflection should be allowed so that; 



542 ELECTRIC TRANSMISSION OF POWER. 

as the wire contracts in cold weather, the above minimum 
should not be passed. 

The total length of wire in the catenary is approximately 

X. = X + g, <3, 

or transposing for the value of d, 

wherein U is the actual length of wire, and L the span. 

From these formulie we can figure d for any temperature. 

The coefficient of expansion of copper is .0000095 of its 
length per degree Fahrenheit, so that we can get at once the 
length for any temperature. 

If the wire we are considering is strung at 75° F. and is to 
encounter a .minimum temperature of — 5° F., enough deflec- 
tion must be allowed at the former temperature to bring the 
deflection at —5° F. to the value just obtg-ined. The length 
of wire at the lower temperature is from (3), 

L' = 100 + ^4^ = 100.0096. 

At 75^ F. this length w^ould be increased by 100.0086 X 
.0000095 X 80 ft., and hence the new value of U would be 
100.076 ft. The deflection corresponding to this is found from 
(4) as follows: 



d = \/' 



300 ^^6 ^1.69 ft. = 20.28 inches. 



A large allowance in deflection must, therefore, be made for 
such variations in temperature as are likely to be encountered 
in northern climates. 

The changes in deflection due to changes of temperature are 
found in practice to be somewhat lessened by the fact that the 
wire as strung is under tension due to its weight, which modi- 
fies its expansion and contraction. The actual coefficient for 
copper wire under various tensions has never been properly 
investigated. It undoubtedly is subject to considerable vari- 
ations, and .000005 \-o perhaps a fair approximation. 



LINE CONSTRUCTION. " 643 

This matter of temperature is unfortunately not all that 
must be looked out for. We have fully taken care of the 
weight of the wire itself, but it is exposed to other and some- 
times dangerous forces in the weight of the ice coating that 
is to be feared in winter, and the strain of wind pressure on 
the wire either bare or ice-coated. 

Taking up these in order, let us suppose the wir^ to become 
coated with ice to the thickness of half an inch, quite a pos- 
sible contingency in severe winter storms. A layer of ice of 
this thickness would weigh 0.54 lb. per linear foot, thus loading 
the wire with more than its own weight. Assuming this load 
at the minimum temperature of —5° for which the assumed 
deflection was 0.57 ft., the tension of the ice-loaded wire be- 
comes from (1), 

^^ (1^X^^2,051 lbs. 
8 X .57 ' 

This is dangerously large, far beyond the elastic limit of the 
wire, and more than likely to bring down weak joints. 

And beyond this the wind pressure must be considered. 
This may be taken as acting at right angles to the weight of 
the wire and adding materially to the resulting total stress. 
The total pressure P on a wire is, per foot, approximately 
P ■■= .05 p D, where p is the normal pressure of the wind per 
square foot, and D is the diameter of the wire in inches, p 
varies from a few omices per square foot in light breezes to 40 
or 50 lbs. in a hurricane. 

Assuming 40 lbs. as the greatest pressure likely to be en- 
countered, we can at once find its effect on the line under con- 
sideration. For our No. 00 wire, 

P = .05 X 40 X .364 = .728 lbs. 

This pressure is combined with the weight of the wire as a 
force acting at right angles; hence the resultant stress, which 
we may call W, is 

W = ^w^ + P2 = V(.4)2 + (.728)2 = .83. 

This, from the example given, is obviously a dangerous strain 
on the wire. But the combination of even half the normal 
wind pressure just assumed with an ice-coated wire would be 



644 ELECTRIC TRANSMISSION OF POWER. 

disastrous. Taking the ice as half an inch thick as before, 

D = 1.36 P = .05 X 20 X 1.36 = 1.36, 
and 

W - V(.94)2 ^ (1.35)2 _ 1^65, 

Substituting in (1), 

8 X .57 ' 

This is over the breaking weight of the wire, which must con- 
sequently give way, and would almost infallibly wreck the line 
in so doing. This means that the factor of safety of 4, assumed 
at the start, is too small for due security. It is sufficient for a 
moderate climate, where high winds are rare, but 5 is generally 
preferable, while 7 or 8 should be used in cold and exposed 
regions. It must be remembered that joints are weak points 
in the wire; a carefully soldered Western Union joint has only 
about 85 per cent the strength of the wire. Fortunately, trans- 
mission lines seldom accumulate half an inch of ice. One- 
quarter of an inch is an unusually thick coating, and with very 
high-tension lines there seems to be a tendency to check the 
formation of a sleety covering. For extreme tensions a 
stranded conductor of hard-drawn copper is advisable as being 
more reliable than a single wire, and possessing a much higher 
available elastic limit. 

The same process that served to take account of an ice coat- 
ing, i.e., adding the distributed load to the weight of the wire, 
can be readily applied to finding conditions of safety in the use 
of bearer wires carrying the conductor suspended from them. 

An interesting corollary to these computations is finding the 
maximum length of span which can safely be used in an emer- 
gency such as crossing a river or canon. Suppose we use 
simply hard-drawn copper wire of the same size as before. Its 
ultimate tenacity is about 6,270 lbs. Using it with a factor 
of safety of 6, the permissible value of T becomes 1,045 lbs. 
W is as before 0.4 lb., and we will assume that for the purpose 
in hand the wires are spread and the deflection is permitted to 
be 10 ft. From (1) we have for the permissible length of span 

'^. (6) 



'/ 



LINE CONSTRUCTION. 545 

Substituting the above values of the known quantities, we have 



-v/ 



8 X 1045 X 10 ,.„,.. 

= 457 + feet. 

.4 



Ten, however, is a preferable factor of safety, which corresponds 
to a length of span of 354 ft. In extreme cases a bearer of 
steel cable may be used, of the highest available tenacity, and 
carrying the copper line wire to secure the requisite conduc- 
tivity, or a steel or silicon bronze wire may be used alone; the 
conductivity being made up elsewhere in the line to the de- 
sired general average. The steel is rather the more reliable of 
the two, but is more likely to deteriorate through rusting. An 
ultimate tenacity of 150,000 lbs. per square inch is the limit for 
either material, with factor of safety of 10 for practical working. 
Now assuming No. 00 silicon bronze or its equivalent in steel 
cable and the same factor of safety as before, the working ten- 
sion rises to 2,612 lbs., and allowing 20 ft. deflection, the pos- 
sible length of span is 



'S/ 



8 X 2612 X 20 . ^„^ „ , 
— -: = 1,022 + feet. 



Spans of even this length can be managed without any very 
elaborate terminal supports. When the line wires are heavy 
and numerous, or longer spans must be used, it may be neces- 
sary to use stout bearer cables, arranged like a rudimentary 
suspension bridge with a footpath, to facilitate inspection and 
care of the conductors. The expense of such a structure is 
sometimes justified by enabling one to avoid long and expen- 
sive detours. When a simple long span of conductors is used, 
the support of the ends and the proper insulation of the tense 
wires require care. A timber truss well guyed will answer 
in most cases, and the strain may be distributed among several 
stout insulators. The conductors should always be in duplicate 
across such a span. Increasing the deflection is the simplest 
and most effective way of securing a proper factor of safety 
in the conductors. Line construction for power transmission 
was originally patterned after the construction usual with 
telephone and telegraph wires, and followed with little modi- 
fication in early electric light plants. The spacing of the poles 



546 



ELECTRIC TRANSMISSION OF POWER. 



follows the precedent established for poles loaded with cross 
arms and wires strung so close that it was necessary to pull 
the wires taut to avoid fouling them. There is no reason for 
using anything but very moderate tensions on lines with the 
wires spaced 3 to 6 ft. apart. 

Of course, to the eye of the old telephone constructor, a 
line with large deflections looks very slip-shod, but actually 
it is far safer and more desirable than a taut line. From 




0.3 0.4 0.5 0.6 0.7 

percent increase in length of conductor 
Fig. 265. 



the properties of the catenary, it follows that tension in- 
creases very rapidly as the deflection decreases, while the 
length of the conductor between supports changes very little. 
Fig. 265 shows graphically the relations between the deflec- 
tion as a fraction of the span, tension in terms of the weight 
of conductor in the span, and variation in the length of the 
catenary. It will at once be seen that in reducing the de- 
flection below about 2 per cent of the span length, the ten- 
sions increase very rapidly, while the change in the length of 
the conductor is very trifling, hardly more than a few tenths 
of a per cent. 

It pays, therefore, to use fairly large deflections in all cases 




PLATE XXI. 



LINE CONSTRUCTION. 547 

where the Hne is exposed to severe strains. The curves of 
Fig. 265 give sufficient data for the solution of most Hne prob- 
lems, save in the case of very long spans in which generally 
copper would not be used, and which demand precise calcu- 
lation. 

One need merely make the span tight enough to avert risk 
of swinging together, and to keep the wires from being too 
near the ground at the centre of the catenary. Any more 
than this is a concession to appearances, harmless enough if 
not carried too far. Deflections of 2 to 3 per cent of the span 
at normal temperatures are none too great for most situations. 

Bearing in mind that the variation in the length of con- 
ductor between supports changes with the temperature to the 
extent of probably about one one-hundredth per cent for each 
20° F. when under strain, one can quickly approximate the 
variations in the factor of safety from Fig. 265. Of course 
there are small variations in the strength and elasticity of the 
wire which might be taken into consideration, but there is so 
much imcertainty about the actual coefficient of copper wire 
when changing temperature under strain, that the most one can 
do is to keep on the safe side, perhaps even to the extent of 
using the coefficient imreduced for strain, which then amounts 
to about one-hundredth per cent elongation for 11° F. In the 
140-mile transmission of the Bay Counties Power Co. to Oak- 
land, Cal., an extremely long span became necessary in cross- 
ing the Straits of Carquinez. The problem was to cross a 
deep, swift, navigable waterway, 3,200 ft. wide at the narrow- 
est point. Submarine cables were out of the question, and 
the United States Government required 200 ft. above high-water 
mark for the low^est point of any suspended structure. 

On the north shore, on a point 160 ft. above high water, was 
erected the skeleton steel tower shown in Plate XXI. On 
the south shore there was higher land, and a similar tower 
65 ft. high sufficed. The construction adopted was that gen- 
erally used for steel tower work; and each tower bore near its 
top, four massive wooden out-riggers surmounted by the insu- 
lated saddles that carried the weight of the cable spans. 

As in suspension bridge work, the cables rest upon rollers 
upon the saddles, and then extend far shoreward to the anchor- 



548 



ELECTRIC TRANSMISSION OF POWER. 



ages, where the strain is taken. From anchorage to anchor- 
age the span is 6,200 ft. Each cable consists of nineteen 
strands of steel, galvanized, is seven-eighths of an inch in dia- 
meter over all, and has the electrical conductivity of No. 2 
copper. The breaking strain of the cable is 98,000 lbs., each 
span weighs 7,080 lbs., and, as suspended with 100 ft. dip, has 
a factor of safety of 4. 

Two difficult problems of insulation were presented. First, 
the great weight of the cable must be supported at the saddle 




Fig. 266. 

with insulation adequate for 60,000 volts. Second, the pull 
must be taken at the anchorage with equally high insulation. 
The pull of the cable being 12 tons, the task at the anchorage 
was by far the more difficult of the two. 

At the saddle the weight is taken upon huge triple petticoat 
porcelain insulators, each built up of four great nested porce- 
lain cups, the inner being filled with sulphur, securing a large 
steel pin. Six such insulators, each 17 in. in diameter over the 
outer petticoat, cooperate to sustain the pressure at each 
saddle. Fig. 266 shows a cross section through insulators, 
supports, frame, and saddle. The heads of the insulators are 



LINE CONSTRUCTION. 



649 



built into a timber platform which serves at once as a rain 
shed and a base for the cast-iron saddle proper. This carries 
in line five steel grooved sheaves over which the cable passes. 
Fig. 267 shows the structure in longitudinal section, together 
with the suspended platform beneath it, for ease of access. 

The strain insulators for the anchorage are of highly ingen- 
ious construction. Micanite seemed to be the only insulating 
substance possessing the necessary mechanical strength, and 
to prevent surface leakage across it the surface exposed to 



Each layer fastened wH^ 
wooden dowel-pins 




Fig. 2G7. 



leakage was enclosed in an oil tank. Fig. 268 shows the struc- 
ture of the completed insulator more plainly than description. 
Two of these insulators are put in series and enclosed in a 
shelter shed to keep off water, for each cable, the pair being 
secured to a long tie rod anchored in a massive bed of concrete. 
Great care was taken m all the details of the structure to 
secure all the insulation practicable, even the timber out- 
riggers carrying the saddle insulators being filled and varnished, 
and the foundation timbers proper being boiled in paraffin. 
The use of four cables gives one reserve conductor in case of 
accident. The total length of the span from tower to tower 



550 



ELECTRIC TRANSMISSION OF POWER. 



is 4,427 ft., 2| times the span of the Brooklyn Bridge. It is 
one of the most striking engineering feats in the records of 
electrical power transmission, and has withstood successfully 
very severe tests. 

Such structures are necessarily costly, but they are more 
reliable than submarine cables and cheaper than long detours. 
It should be noted that the deflection of the cables is over 2 
per cent of the span length, and that even so the factor of 




Fig. 268. 

safety is not so great as is generally advisable. But this 
merely means that the span is nearing the maximum lengtji 
advisable without a greater deflection, which could well have 
been given had it been necessary, by making the towers some- 
what higher. It represents a rather extreme but necessary 
construction, and has done its work admirably. 

When bodies of water too wide for a suspended structure 
must be crossed, there is trouble ahead. In marshy shallows 
a timber trestle is perhaps the best way out of the difficulty, 
but in deeper water cables may occasionally have to be used, 
although rarely in view of the possibilities of very long spans 
like the one just mentioned. 

Cables can be obtained that will stand 5,000 to 10,000 volts 
alternating current under water with a fair factor of safety. 
Above this pressure success is problematical. Near the ends 
of the line before the raising or after the reducing transformers, 
cables may be successfully used ; but when the obstacle is in the 
middle of a long line, the choice is between evils, reducing the 
pressure locall}^ by an extra transformation, or going the long 
way around. Either expedient is costly and to be avoided if 
possible. It is almost needless to say that when cables are 
used they should be in duplicate. 



LINE CONSTRUCTION. 



661 



POLES. 

As a rule, all aerial lines in this country are carried on wooden 
poles. Iron poles are used much for railroad work, and abroad 
considerably for miscellaneous work, including power trans- 
mission. 

GENERAL DIMENSIONS OF POLES. 



Total 

Length in 

Feet. 


Diameter 
at Top - 
Inches. 


Diameter 6' 
from Butt, 
in Inches. 


Depth of 

Setting 


Approximate 

Weight, 
in Pounds. 


Number that Can 
Be Loaded on 
a Pair of Cars. 


35 


n 


m 


5-6'' 


650 


90 


40 


7i 


13 


6' 


900 


75 


45 


8 


14 


e'6'' 


1,000 


65 


50 


8 


16 


r 


1,300 


50 



In the eastern and central parts of the United States, white 
Northern cedar, chestnut, and Northern pine are the most 
desirable woods for poles, in the order named. West of the 
Rocky Mountains, redwood is a favorite, and stands even ahead 
of cedar in estimation. Abroad, Norway fir is highly valued. 

For power transmission work the poles should be both long 
and strong — long to carry the wires well out of reach and 
often above other circuits; strong to stand the pressure of the 
often heavy wires and the wind. In open country the length 
is less important, and it is sometimes well to use rather stubby 
poles, say not over 35 ft., but extra stout. The poles should 
be straight and free from laiots, of sound, live wood, and the 
bark should be peeled and the poles trimmed and shaved. 

The foregoing table gives the size and other characteristics 
of the poles most likely to be used on power transmission work. 
This is based on cedar poles, and the dimensions given are the 
minimum to be p^mitted in first-class line construction. 
Pine and redwood and chestnut are somewhat lighter than 
poles of the weight given. For the best utilization of the 
lumber, the top diameter of the pole should be about f of the 
diameter at the ground. Natural cedar poles commonly show 
rather more taper than this, natural chestnut poles rather less. 



652 ELECTRIC TRANSMISSION OF POWER. 

It will be noted that poles of these lengths have generally to 
be carried on two cars, one being too short. Various preserva- 
tive processes are used to increase the life of wooden poles. 
Of these, ''creosoting" is generally preferred. The process 
consists of stowing the poles in an air-tight iron retort, treat- 
ing with dry steam for several hours, and then forcing in the 
preservative fluid, preferabl}^ tar oil from coke ovens, under 
heavy hydraulic pressure. Creosoting is more effective on 
open-grained timber than on harder woods, and when properly 
performed will give a pole life three or four times longer than 
if untreated. The process does not weaken the wood unless 
the preliminary steaming is at too high temperature or too long 
continued. Cross arms, pins, and the like are best treated by 
the vacuum process at a moderate temperature. 

When not specially treated, the poles should be coated 
heavily with pitch, tar, or asphalt on the portion to be buried 
up to and fairly above the ground level. 

The pole top is usually wedge-shaped or pyramidal, and this 
roof should be painted or tarred. Before the pole is erected, 
the gains for the cross arms are cut, and the cross arms them- 
selves should be bolted in place and the pins set for the insu- 
lators. The upper cross arm centre should be 10 to 18 inches 
below the extreme apex of the pole, and the lower cross arms 18 
to 36 inches further down. In power transmission work em- 
ploying heavy wires, the spacing of the cross arms should be 
guided by the arrangement of the circuits, there being no 
standard practice. 

The cross arms themselves are of wood, having the same 
characteristics of strength and durability as the poles; hard 
yellow pine being rather a favorite. They are, of course, of 
such length as the work demands; in power work, generally 
from 4 to 8 ft. There are two sectional dimensions in common 
use, 4^ X 3J in., and 4| x SJ in., also a 4 X 5 in. section for heavy 
work. The latter should be used for the longer cross arms and 
those carrying heavy cables or the like, while the former serve 
for 4 or 5 ft. arms not heavily loaded. The cross arms are best 
secured in their gains by a strong iron bolt passing through both 
the pole and the cross arms in a hole bored to fit, and set up 
hard with wide washers under head and nut. This construe- 



LINE CONSTRUCTION. 55B 

tion makes a cleaner job than the practice of fastening the 
cross arm with two lag screws, and permits of easier changes 
and repairs. The bolt should be about three-quarters of an 
inch in diameter, and the gain is from 1 to 2 in. deep, according 
to the size of the pole. Lag screws are cheaper, however, and 
are, as a rule, employed in ordinary work. Cross arms 6 ft. long 
or more should be braced. 

In ordinary transmission circuits about 50 poles per mile 
are used, 110 ft. apart, or 48 per mile, being a common spacing. 
The setting should be carefully done. The earth should not be 
disturbed more than enough to make easy room for the pole, 
and the earth and gravel filled in around the pole should be 
heavily tamped. When setting poles in soft ground, it is some- 
times impossible to give them stability enough merely by tamp- 
ing, and the best procedure is to fill in concrete about the pole, 
using one part of Portland cement to three or four parts of sand 
and heavy gravel or broken stone. 

The stresses to which a pole line is exposed may be classi- 
fied as follows: 1. The direct weight of the wire and the down- 
ward component of the wire tension. 2. Bending moment due 
to the pull of the wires at turns in the line. 3. Wind pressure 
on poles and wires. 4. Wind pressure plus ice. 

1. In power transmission lines built as has been indicated, 
the crushing stress is completely negligible. The ultimate 
resistance against crushing amounts in the woods used for poles 
to at least 5,000 lbs. per square inch. The ordinary pole, there- 
fore, has a factor of safety of several hundred, and the danger 
of crushing, even from tense and ice-laden wires, has no real 
existence. 

2. Bending moment is more serious, since the forces acting 
have a long lever arm. The ultimate effect of this stress is to 
break the pole, generally near to the surface of the ground, by 
crushing the fibres on the side next the stress and pulling apart 
those on the other side. The pull or push necessary to break 
a round pole by bending is approximately 

where A is the area of the pole section at the ground, S the 



554 ELECTRIC TRANSMISSION OF POWER, 

strength per unit area, R the radius at the ground, and D the 
distance between the ground and the centre of pressure. 

For example, take a 40 ft. pole, 13 in. in diameter at the 
ground. Taking S = 7,500 lbs. per square inch and the centre 
of pressure as 32 ft. above the ground, (6) becomes 

_ 132 X 7,500 X 6.5 _ 
- ^- 4X12X32 -4'1891bs. 

The factor of safety allowed should be never less than 5, and up 
to 8 or 10 in cases where high winds are to be expected. Square 
sawed poles are relatively weaker than natural poles, and may 
be approximately figured by the same formula, taking R as half 
the side at the ground. The values of S are rather uncertain, 
but the figure given is about right for the woods customarily 
used in large sticks. Small samples run relatively higher 
from being of selected material. 

The following table gives the commonly received tensile 
strengths for the American woods generally used in electric 
construction, the figures being derived from small samples,, and 
hence to be taken with reservations in the case of poles, while 
fairly applicable to cross arms and pins. 

wT^r^A Value of S per 

W°«^- square incL 

Cedar 11,000 

Chestnut . 10,000 

Yellow Pine 12,000 

Hickory . . . 14,000 

Redwood 11,000 

White Oak 14,000 

Locust 20,000 

Practically, poles at angles should always be guyed, like ter- 
minal poles. This is best done with a steel rope one-quarter 
to one-half an inch in diameter, taken from as near the centre of 
the stress on the pole top as the position of the circuits permits. 
The guy rope should extend downward at an angle of from 45° 
to 60° with the pole, directly back from the direction of the 
pull on the pole, and should be drawn taut and securely fas- 
tened to a tree or a firmly set post. Where there are three or 



LINE CONSTRUCTION, 555 

four cross arms, what is known as a Y guy is often used, con- 
sisting of a guy rope attached near the pole top and another 
just below the cross arms. These divide the tension and are 
moored by a single guy rope in the ordinary manner. This 
arrangement is not connnonly needed in transmission work 
save when the circuits are numerous or the strain exception- 
ally severe, and in any case great care should be taken to keep 
the guy wires well clear of the high-voltage lines. Sometimes 
two or more light guys m different directions are valuable in 
securing a pole, when proper setting is very difficult, and may 
save expensive blasting. 

The bending moment due to an angle is normally 2 T cos 

^ where T is the tension as already determined and a is the 

angle made between the wires at the turn. For the simple 
circuit of No. 00 wire already discussed and a turn with 120° 
between the wires, taking a factor of safety of 7 on the wire, the 
tension per wire is 507 lbs. The total pull for the two wires 
forming the circuit is then 2,028 lbs. x cos 60° = 1,014 lbs., 
a pressure rather greater than would be permissible without 
guying. 

3. The wind pressure on the wires has already been com- 
puted, and the same formula serves for figuring the pressure 
on the poles, using the mean diameter in inches, and for the 
total pressure, multiplying by the feet of pole exposed. For 
example, assuming a pole of 34 ft. out of ground, 7 in. diameter 
at the top and 13 in. at the ground, the average diameter is 
10 in , and for a storm giving a normal wind pressure of 40 lbs. 
per square foot, 

P = .05 X 40 X 10 X 34 = 680 lbs. 

This acts virtually at the middle point of the pole, hence it is 
equivalent to 340 lbs. at the pole top, to which must be added 
the pressure on the wire itself, which for the circuit in ques- 
tion amounts to about 145 lbs. more, making a total of 485 lbs. 
This is well within the safety limit, and would remain so even 
if there were half a dozen wires instead of two. As 40 lbs. per 
square foot is an extreme wind pressure, never met in most 
localities at all, it is safe to say that a well-set line of the poles 



556 ELECTRIC TRANSMISSION OF POWER. 

assumed, loaded with any power transmission circuit likely to 
be met in practice, is perfectly secure so far as wind pressure 
alone is concerned, unless the line is literally struck by 
a cyclone. 

4. The most dangerous stresses on an aerial line come from 
sleet storms that load the wires with ice, increasing the weight 
and the lateral thrust due to wind pressure. On rare occasions 
ice may be formed on wires to the depth of a couple of inches. 
Such a coating on a No. 00 wire would weigh about 5.9 lbs. per 
lineal foot. The mere w^eight of this would produce a ten- 
sion, assuming d = 2 it., and No. 00 wire as before, 

y = ^^^^^^^ = 4,000 (yei-y nearly), 

which is well above the tensile strength of the wire if soft- 
drawn. Allowing a wind pressure of 20 lbs. per square foot, 
the pressure on a single span of 100 ft. would be 

P = .05 X 20 X 4 X 100 = 400 lbs. 

Adding to this 170 lbs. pressure on the pole itself, the total for 
a single circuit of 2 wires would be 970 lbs. total thrust, which, 
while high, is not likely to carry down the pole. Even 6 No. 00 
wires would give a total thrust of only 2,570 lbs., which is still 
below the ultimate strength of the pole. The pole line is there- 
fore stronger than the wires. If a line is to stand such extreme 
stresses, which are far beyond really practical requirements, 
the only safe plan would be to string hard-drawn wire, shorten 
the poles and increase the diameter, and guy frequently. As a 
matter of fact, the insulators and their pins are quite sure to 
give way before the wires or poles under these extreme stresses, 
and in most transmission lines are the greatest source of 
anxiety. 

The insulators themselves can be made strong enough to 
stand the greatest stresses to which they will be subjected, but 
it is not easy to so support them as to give ample strength 
without endangering the insulation. The ordinary wooden 
pin answers well if the circuits are not very heavy or likely to 
be weighted with ice. 

By common consent, locust is the wood best suited for pins. 



LINE CONSTRUCTION. 



557 



which for general Hne work are about 12 ft. long and 2 in. in 
extreme diameter at the shoulder, below which the pin is 
cylindrical and U in. in diameter. This fits a hole bored in 
the cross arm and is secured by a nail driven through arm and 
pin. The top of the pin is threaded for the insulator to be 
used. Under extreme forces these pins are liable to break at 
the shoulder; and for transmission circuits carrying very 
heavy wire, for long spans, and for cases where special insula- 
tors demand extra long pins, a variation of this construction 




Fig. 269. 



is desirable. On the Pacific coast excellent results have been 
obtained from eucalyptus pins, which are even tougher and 
stronger than locust, but unfortunately not readily obtainable 
in the East. Lacking both locust and eucalyptus, a fair pin 
may be made from seasoned oak. Pins for heavy transmis- 
sion work may with advantage be made much heavier than 
ordmary up to 2^ in. at the shoulder and up to 2 in. in the 
cylindrical base, the standard pin-hole in the corresponding 
insulators being If in. 

In ordinary line work, the pins are set 12 to 14 in. between 
centres. With heavy wires this distance may advantageously 
be increased to 18 to 24 in. At very high voltage these dis- 
tances must be increased farther, perhaps up to 48, 60, or 



558 



ELECTRIC TRANSMISSION OF POWER. 



sometimes to 72 inches and more in dealing with voltages in 
the uncertain region beyond 50,000 volts. 

When the lines have to be transposed, as in long parallel 
alternating power circuits, this transposition involves some 
careful work, for the wires must be kept well clear of each 
other. Heavy strain pins will generally answer the purpose 
and allow the transposition to be safely made. Such trans- 
position should not be made at an angle or elsewhere where 
the tension on the insulators is unusually great. 




Fig. 270. 

A good example of line construction for heavy transmission 
work is found in the line constructed a few years ago for the 
Niagara-Buffalo power circuit. Fig. 269 shows the pole head. 
The cedar poles, intended ultimately to carry 12 cables each 
of 350,000 cm., are extra heavy ; varying from 35 to 50 feet 
in length with tops 9 and 10 in. in diameter. The two maui 
cross arms are of yellow pine, 12 ft. long and 4 X 6 in. in sec- 
tion, fastened to the pole with long lag screws, and braced by 
an angle iron diagonal ^ X 2| in., bolted to the pole and to 
the bottom of the cross arm at each side. Each side of each 



LINE CONSTRUCTION. 559 

arm is bored for three pins spaced 18 in. apart. The trans- 
mission is three-phase, and one complete circuit is on each side 
of each cross arm. The cross arms themselves are 2 ft, apart. 

The pins and insulators, Fig. 270, are special, the pine being 
much heavier than usual, and the insulators of dense porcelain 
formed in the usual double petticoat design. They have one 
peculiar feature: a gutter is formed on the external surface, 
leading to diametrically opposite lips so placed as to shed 
dripping water clear of the cross arm, thus lessening the dan- 
ger of ice formations. Each of the main circuits is designed to 
transmit 5,000 HP. A short cross arm below the others carries 
a private telephone line. The right of way is in part owned by 
the operating company and fenced in, and in part along the 
Erie Canal. The line is elaborately transposed every five 
poles to annul induction. So frequent transposition is un- 
usual and generally needless. Transposition every 20 to 40 
poles is ample for ordinary cases, and on long lines in open 
country it is enough to transpose once in a couple of miles. 

This line is admirably constructed, but it is a grave question 
whether all the circuits should be carried on a single pole line 
on account of the difficulty of executing repairs, and the insu- 
lators are rather closer to the cross arms than seems safe in 
view of the climate and the high voltage to be employed. Cer- 
tainly at voltages above 10,000 a duplicate pole line is prefer- 
able to running two circuits on one pole line. It is, however, 
entirely feasible to execute repairs on one side of a pole like 
Fig. 269 while the circuit on the other side is in use, although 
it is a careful job, and should not be attempted unless, as in 
this case, the cross arms are unusually long. 

Another admirable type of high-tension line construction 
is found in the lines of the Missouri River Power Co., of which 
Fig. 271 shows the pole head and detail of pin and insulator 
construction. This line, it should be said, is 65 miles long, 
and has been in regular service for four years at 57,000 volts 
with unusual immunity from interruptions of service. 

The poles are of cedar, varying from 35 to 75 ft. according to 
the necessities of the case, with tops from 9 to 12 in. Poles are 
normally spaced 110 ft. The cross arms are of Oregon fir, and 
the pins of oak boiled in paraffin. The insulators are glass^ 



560 



ELECTRIC TRANSMISSION OF POWER. 



with an additional glass sleeve surrounding the pin almost 
down to the cross arm. An especial feature of this system is 
the use of white oak braces for the cross arms instead of the 
usual metal, the change being in the interest of insulation be- 
tween wire and wire. 

This principle has been carried still further in the long trans- 
mission line from Logan to Ogden and Salt Lake City, Utah, 
in which case the entire pole construction is wood, the cross 
arms being mortised through the pole. Locust pins, paraffin- 
treated, are used of extra length so as to carry the insulators 





Fig. 271. 

well above the cross arm. The change from metal braces and 
cross arms was made as the result of bitter experience, it hav- 
ing been found that in wet weather these metal parts became 
the seat of trouble by burning the adjacent wood, especially 
in case of a broken insulator producing considerable leakage to 
the cross arm. On the other hand, iron braces and iron or 
steel pins are in common use on some very large high-voltage 
systems with apparently excellent results. 

At times wooden pins have given much trouble from burn- 
ing, owing to leaky insulators, and show a strong tendency to 
"mould" and soften in the thread and at the cross arm. This 



LINE CONSTRUCTION. 561 

has been traced to the action of the brush discharge at high 
tension, probably setting free nitric acid from the moisture 
present. In certain locaUties a combination of moisture and 
dirty insulators has been very destructive of wooden pins, in 
one plant causing 26 shut-downs from burning, in a single 
month. The truth seems to be that untreated or imperfectly 
treated wood may be rapidly attacked in a moist atmosphere 
by the discharge at high voltage over dirty insulators. With 
thoroughly treated wood this difficulty disappears. On the 
Logan line referred to, the pins are treated as follows. The 
locust pins are heated with stirring in vats of hot paraffin at 
150° C. for 6 to 12 hours, and then are kept submerged during 
gradual cooling. Thus treated, the paraffin saturates the pins 




Fig. 272. 

clear to the core, and they give practically no trouble from 
burning or ''moulding." There seems to be no good reason 
why a pin thus treated should not stand up well in almost any 
climate. 

Steel or iron pins, however, are very advantageous in the 
matter of strength, and give admirable service. They are sub- 
ject to the difficulty of putting severe strain on the insulator 
thread if used alone, so that it is desirable to use a lead bushing 
around the steel pin to furnish the thread, or otherwise to 
interpose soft material. Steel pins are now made with sleeves 
of treated wood for the thread portion, and with porcelain 
sleeves covering the shaft of the pin clear down to the cross 
arm after the idea of Fig. 271. Such a composite pin is shown 
in Fig. 272. The wood, of course, may be replaced by lead if 
anybody objects to wood, and the porcelain sleeve retained. 

As between wooden and steel pins, the mechanical advantage 
when the strains are severe is with the latter, both on account 



662 ELECTRIC TRANSMISSION OF POWER, 

of intrinsic strength and less wea .ening of the cross arm on 
account of smaller diameter. Electrically, the advantage lies 
on the side of well-treated wood. Either can and does give 
good service at the highest voltages yet employed. 

Much the same sort of question arises as between wooden 
and iron poles and cross arms. Iron poles are considerably 
used abroad, although not on such high voltage as is used in 
the large American systems. They are excellent mechanically, 
and have a very long life. On the other hand, they cost sev- 
eral times as much as wooden poles and when used with iron 
cross arms as usual, carry the earth potential squarely up into 
the interior of the insulator itself. Any failure of the latter 
means an instantaneous and complete shut-down of the line, 
which is a very serious contingency. 

As a rule, failure of an insulator on a wooden pole line does 
not do this. It may cause progressive burning and leakage, 
which gives warning of trouble and eventually may become 
serious, but often gives ample opportunity for temporary re- 
pairs before it puts the line out of service. With these condi- 
tions it seems like taking unwarrantable chances in the present 
state of insulator construction, to replace wooden poles by 
iron in the ordinary form of line construction. 

An altogether different question is raised by the introduction 
of the tower construction which has been in successful use for 
a year or so in the Guanajuato transmission plant in Mexico. 
The plan here followed was to employ steel towers of sufficient 
height and stability, not only to replace wooden poles but to 
admit the use of very long spans, thus greatly reducing the 
number of insulating supports which are by common experi- 
ence, the weakest points in the line. The tower and tower head 
is shown in Fig. 273. The structure is a standard 45 ft. gal- 
vanized steel windmill tower anchored at each corner to a 
concrete foundation. The span employed is about 440 ft., the 
conductors being of hard-drawn copper cable equivalent to No. 1 
B & S. The deflection in the centre of the span is variously 
stated at from 7^ to 18 ft. At the former figure, the factor of 
safety would be less than 3 as regards the ultimate strength of 
the cable, and less than 2 on the elastic limit. At the latter 
figure the conductors would be less than 20 ft. above the ground 



LINE CONSTRUCTION. 



563 



at the centre of the span. The actual deflection is probably 
intermediate between the reputed figures. 

These towers cost laid down from Chicago between $60 and 
S70 each, and from 9 to 12 would be required per mile. Assem- 
bling and erecting amounted to about $7 each, brmging a con- 



\4'5.5'^Ch»iiBel 




2x2x)gL 



^\ 



-This pipe screws into 
casting at top of tower 



Fig. 27." 



servative estimate of the line structure to about $750 per mile, 
including insulators. 

The construction is a very ingenious one, and possesses great 
convenience for regions where poles are scarce or where the 
ravages of insects are to be feared. If constructed, however, 
with the factors of safety generally to be recommended in over- 
head lines, the cost would run materially higher than that just 



564 ELECTRIC TRANSMISSION OF POWER. 

stated, by reason of the use of more or higher towers, to allow 
shorter spans or more deflection. Employed with caution the 
plan is mechanically good. Electrically, it is open to the ob- 
jection to all metal structures of complete dependence on the 
insulators, in this case subject to more than usual strain. 

The Guanajato line has suffered severely from lightning, and 
the installation of lightning rods on many of the towers did not 
prove an adequate remedy. A grounded cable stretched above 
the transmission wires coupled with replacement of all the origi- 
nal insulators by larger ones has helped materially. It would 
appear necessary on steel tower lines to take extraordinary pre- 
cautions to ensure a large factor of safety in the insulators. 

The reduction of the number of insulators is a material gain 
over ordinary practice. It should, however, be pointed out 
that the long-span prniciple in a less extreme form can readily 
be carried out with wooden poles, employing conservative 



Fig. 274. 



factors of safety and still giving a material gain in cost and in 
the number of insulating supports. 

A stout 40 or 45 ft. pole will carry 3 cables, such as are used at 



LINE CONSTRUCTION. 565 

Guanajuato, on a spacing of, say, 20 poles to the mile with a 
considerable improvement in the factor of safety and at about 
half the cost of construction under ordinary conditions. The 
difference in annual charges on the cost is great enough to pro- 
vide for the replacement of wooden construction every dozen 
years, even assuming eternal life for the steel. 

Barring local conditions, modifying considerably the rela- 
tive costs, or affording special reasons for discrimination, ex- 
pense is about a stand-off between the two, save as a low first 
cost is sometimes important. 




Fig. 275. 

Insulators for high-tension work are now generally of porce- 
lain, although glass is being successfully used, as in the Missouri 
River Power Co. plant just referred to, and in the great Utah 
system. The form of insulator used in the latter case is shown 
in Fig. 274. It is only 7 in. in diameter, but with the long 
paraffined pins and wooden construction there used, it has 
given good service for half a dozen years past at 40,000 
volts. 

Porcelain insulators, although more costly than glass and 



566 ELECTRIC TRANSMISSION OF POWER. 

requiring individual testing, are mechanically stronger, and 
probably stand weathering more successfully. As has already 
been indicated, the path of the discharge when an insulator 
''spills over" is from edge to edge of petticoats and thence to 
pin or cross arm by the nearest course, so that, irrespective of 
other things, the insulator must have a long sparking distance 
if it is to be successful at high voltage. With either glass or 
first-class well- vitrified porcelain the insulation strength of the 
material is ample, and insulators rarely fail by puncture unless 



Fig. 276. 

mechanically defective. 

Fig. 275 shows a typical insulator of the kind used for very 
high-voltage. It is made in three pieces to insure proper 
baking of the porcelain, which is difficult in large masses. The 
diameter of the upper petticoat is 14 in., and the sparking dis- 
tance is 8 J in. The test voltage is about 150,000 and the 
line voltage 60,000. 

Fig. 276 shows a somewhat different design of about the same 
dimensions, but with a sparking distance increased to QJ in. 
These big insulators are shipped in pieces and cemented when 



LINE CONSTRUCTION. 



66' 



one is ready to test them prior to installation. Fig. 277 is a 
somewhat smaller design, intended for 50,000 volt work, with 
an upper petticoat 11| in. in diameter and a sparking distance 
of 6| in. All these have pin holes If in. in diameter to take an 
extra heavy pin, either steel wdth bushing, or wooden. An ex- 
cellent example of glass insulator for medium voltages, say up 
to 20,000, is shown in Fig. 278. This is 7 in. in extreme dia- 
meter, with a sparking distance of 3 in., and a If in. pin hole 
like the others. All have top grooves, which are preferable 
for high voltage. 

Now, all these insulators are well made and pretty well de- 




FlG. 277. 



signed, and have been used with success, but none of them has 
a high factor of safety. If one glances at the curve of striking 
distances already given, it is apparent that the sparking dis- 
tances for the insulators are for the higher voltage barely twice 
the possible striking distance of the normal voltage. This is 
not enough, considering the possible rises of potential due to 
surging, resonance, and static effects. To increase the size of 
the insulators means increased difficulty of supporting them, 
increased cost, and greatly increased difficulty in getting first- 
class porcelain. A porcelain or glass sleeve over the pin seems 
a very desirable safety precaution. 



568 ELECTRIC TRANSMISSION OF POWER. 

The factor of safety, as regards both dielectric strength and 
sparking distance, should be raised to not less than 3 and pre- 
ferably higher. 

One of the undetermined points in power transmission is the 
degree of immunity from line troubles which may fairly be 
called successful operation. All systems suffer more or less, 
but it seems to be impossible to give actually continuous ser- 
vice at the higher voltages without duplicate lines. One 
experienced engineer regards the conditions as very good if 
one per cent of the insulators do not have to be replaced yearly. 
If the line were of steel poles, cross arms, and pins, this would 
insure an abundant supply of shut-downs not to be averted 
unless by a complete duplicate pole line, and not certainly 
then. In case of a wooden construction, complete failure 
from broken insulators can nearly always be avoided if a 
spare line is available. 



Fig. 278.. 

All lines alike are liable to be the victims of casual accidents, 
like branches falling or being blown across the circuit, large 
birds flying into the wires, lightning, and all sorts of curious 
and apparently trivial causes. 

The chief trouble, however, is in the failure of insulators 
from one cause or another, and next to that stands lightning. 

Lightning, although fortunately not a continuous risk like 
insulators, is a very dangerous contingency, the more so since 
no system of defence has proved entirely effective. Lightning, 



LINE CONSTRUCTION. 569 

so far as transmission systems are concerned, is of two sorts. 
First in danger comes the lightning discharge, which actually 
strikes, fully or by a branch discharge, the line structure itself. 
This is, luckily, an unusual happening, but a very serious one. 
Second comes the induced discharge along the circuit, due to the 
tremendous effect of a lightning discharge at a greater or less 
distance. These induced discharges constitute the vast ma- 
jority of all so-called cases of lightning upon the circuits. They 
are in effect surges of potential of sometimes very great vio- 
lence, but never comparable with even a minor direct stroke. 

These surges seem to be enormously abrupt, and when 
checked as by an inductance, their sparking power is somewhat 
formidable. It is very difficult to form any proper idea of the 
potentials concerned in an actual lightning stroke, but they 
run to many millions of volts, probably several hundred mil- 
lions at times. The induced discharges which make up prob- 
ably 99 per cent of so-called lightning, are of a very different 
order of magnitude, but they certainly give rise to sparks hav- 
ing striking distances corresponding to voltages up to consider- 
ably above lOOjOOO volts. The majority of such discharges, 
however, are of minor violence, but quite sufficient to puncture 
insulation and cause serious damage to apparatus. 

An actual stroke of lightning upon a line is to be dreaded. It 
frequently shatters insulators and poles, and may break down 
apparatus as well, especially if near the station. It will some- 
times distribute its effect for a quarter mile or so from the 
striking point, doing more or less damage at every pole. It does 
not, upon a wooden pole line, necessarily shut down the line, 
although of course it may do so. A duplicate pole line is the 
best safeguard against lightning, since it is highly improbable 
that in a single storm both lines will be hit hard enough to put 
them out of action. 

That component of a direct stroke which follows the lines 
to the station is like an unusually sev^ere induced surge, and 
must be dealt with as best one can. 

Lightning arresters, so called, are merely devices for giving 
induced or other discharges an easy path to ground, while 
checking the tendency of the line current to follow them. They 
consist essentially of spark gaps connecting the line with a. 



670 



ELECTRIC TRANSMISSION OF POWER. 



ground wire and means for checking the following rush of cur- 
rent from the line. In addition, reactive coils are usually 
placed between the machines and the arresters to check the 
surge, and throw it over the gaps to earth. 

As now generally arranged, lightning arresters consist of a 
series of short spark gaps between metal cylinders, in series 





Fig. 279. 



with resistance enough to attenuate the following line current 
sufficiently to allow the arcs across the spark gaps to go out. 
As the line voltage increases, more and more gaps and more and 



imiilliiiufi 



Fig. 280. 



more resistances are put in series. For convenience, the gaps 
are arranged in groups and assembled as required. Fig. 279 
shows a Westinghouse unit, and a General Electric unit respect- 
ively. The former has six gaps, the latter four and two high- 







o ®} 



^SZ_.'L_ ^" (£!_?- 




PLATE XXII. 



LINE CONSTRUCTION. 



571 



resistance carbon sticks. The Westinghouse Company as- 
sembles its resistances separately. 

Either can be arranged to work double pole for low voltage, 
and the General Electric unit is shoAvn so arranged. Both 
companies use reactance coils next to the apparatus. Fig. 280 
shows the General Electric form of coil. The individual gaps are 
about -3^2" ^^v ^^^ f<^^ high voltage enough units are assembled 
to aggregate the required striking distance. The Westinghouse 




Pig. 281. 

cylinders forming the gaps are made of an alloy which pro- 
duces a non-conductive oxide when vaporized by the passage 
of an arc, while the General Electric cylinders are more massive 
and of somewhat different alloy, believed to act mainly by 
chilling the feeble arc permitted by the series resistances. 
Either is effective under not too severe conditions. 

These components are assembled as convenient, generally 
in regular panels. Plate XXII shows a recent type of West- 
inghouse arrester for moderately high voltage including re- 



5T2 ELECTRIC TRANSMISSION OF POWER. 

actance coil arrester cylinders and resistances. Fig. 281 shows 
a General Electric arrester set for a three-phase line, equipped 
with cut-off switches and static protectors in the form of the 
intermediate high resistance carbons, shown as uniting the 
phases midway the groups of arrester units. The office of 
these is to allow minor rises of potential to be equalized through 
the auxiliary resistances, without having to leap the whole 
series of spark gaps, while a heavy lightning surge will go to 

LINE 




ground in the ordinary way. The arrester of Plate XXII has a 
similar function, the connection being diagrammatically as 
shown in Fig. 282. Minor disturbances are eliminated via the 
high-shunt resistance, the series gaps being proportioned to 
spill over on comparatively small rises above the line voltage. 
Heavy discharges are sent to earth over the full series of 
gaps and the relatively low series resistance. 

The Westinghouse Company also employs, especially for the 
protection of very high- voltage apparatus, a device shown in 
Fig. 283, known as a ''static interrupter." It consists of a 



LINE CONSTRUCTION. 



573 



reactance coil in the line, and a condenser connected between 
the coil and the apparatus. One pole of the condenser is 
grounded, and the other is brought to the coil, both being en- 
closed in an oil-filled case. This device very considerably 
attenuates static waves from any source, the wave bemg, as it 
were, checked by the coil and soaked up by the condenser, 
to be frittered harmlessly away in minor oscillations. 

The devices here described are the best yet devised for deal- 
ing v\-ith lightning. They are undoubtedly effective against a 




Fig. 283. 



very large proportion of the induced class of discharges, but 
any and all protection is liable to failure in case of a heavy 
direct stroke. 

As the line voltage is raised, it becomes increasingly difficult 
to deal with the tendency to short-circuit after a heavy dis- 
charge over the arresters. On the other hand, a system that 
is insulated for 60,000 volts with a factor of safety of, say, 2J 
has a margin of some 90,000 volts insulation to protect 
it against the effect of static surges. Hence, such a system is 



574 ELECTRIC TRANSMISSION OF POWER. 

fairl}^ immune as regards most induced lightning discharges, 
although perhaps in increased danger from direct strokes. 

At the power station and at sub-stations it is wise to install 
as complete lightning arrester systems as the state of the art 
permits. If, as is desirable on long lines, section houses are pro- 
vided with means for cutting out and switching the lines, and 
to serve as headquarters for line inspection, arresters may be 
provided there also. But the custom of installing arresters 
on the line at frequent intervals has been abandoned on account 
of the elaborate nature of the protection required for high vol- 
tage and the likelihood of trouble after a severe discharge. 

Grounded wires stretched above the transmission wires have 
in some cases proved useful, in others, ineffective. Present 
practice tends to their use, especially on steel pole lines in which 
the interior of the insulators is already at earth potential. If 
used at all, they should be of strong stranded steel cable such 
as is used for guy wires, the barbed wire sometimes used being 
too weak for safety. The most that can be said for the grounded 
wire is that it may be of considerable use as auxiliary to other 
lightning protection. 

Experience indicates that the best way of stringing a three- 
phase transmission line is in the usual form of an equilateral 
triangle with the apex uppermost. It is undesirable to run 
more than two circuits per pole line at high voltage; and for 
security, at pressures of 50,000 volts and upwards, it is highly 
desirable to run but a single circuit per pole line unless in very 
large plants. 

Circuits should be transposed at convenient intervals to keep 
down mutual indirection, especially at the higher voltages. 
Practice varies in this respect, from transposing every mile or 
less, to making only a few transpositions in the entire length of 
the line. If section houses are used, these afford convenient 
points for spiralling the lines, which at high voltage is some- 
what troublesome. 

It pays to make a very careful and thorough job of the line 
and to use only the best material. 

The life of a pole line is a varying quantity, according to the 
character of the material and the soil hi which it is fixed. With 
wooden poles the chief danger is of course rotting at and below 



LINE CONSTRUCTION. 



575 



the surface of the ground. If one starts with a dry pole, 
thorough and repeated painting with tar-oil, asphalt, or the 
like, from the butt to a little above the ground line, will greatly 
increase the life of the pole. A well set and treated line should 
last at least 15 years, and if the poles were actually '^creo- 
soted," as railway ties are, this life should be extended for 
another decade. But lines set with green poles in damp earth 




Support ia.WaIl 



are likely to require heavy repairs within 6 or 8 years, if not 
sooner. Thorough treatment at the start and judicious inspec- 
tion are necessary to keep a line in proper shape. The me- 
chanical danger points in a line are changes in direction, 
whether horizontal or vertical, and changes in lengths of span. 
Double arms and proper guying will make these points safe. 

One of the minor difficulties in line work is making a safe 
entrance into the power station and other buildings which the 



576 



ELECTRIC TRANSMISSION OF POWER. 



high-tension hnes have to enter. The chief requirement is 
ample space around the conductors. Up to 10,000 volts or so, 
long porcelain wall tubes set through the walls with a slight 
downward slope toward the outside do very well, the wires 
being supported on each side of the wall by line insulators. 

At really high voltages the striking distance is so consider- 
able as to call for better insulation around the wires, and many 
plans have been tried, mainly based on a wide hole with the 
wire held centrally in it by suitable insulators. Two of the 
best schemes for entrance hitherto tried are those shown in 
Figs. 284 and 285. 

These are nearly self-descriptive. In Fig. 284 the purpose 




X 2 X 2 Marble 
2x 24 Glass. Tube 



of the perforated plate glass cover at the inner end of the 
tile is to keep out the cold. Otherwise, the tile might as well 
be open, and open tiles are frequently used. The little rain 
shed and the downward slope of the wire to keep away 
dripping water are of obvious use. 

Fig. 285 is an excellent construction for cold climates. A 
long tube of high-grade porcelain may well replace the glass, 
and the tube should be given a slight slope as in the previous 
case. 

High- voltage wires should never be brought through a roof, 
or into any contracted place. Allow plenty of space about 
them. Inside the building they are sometimes insulated, but 
should be treated with the same respect as if they were bare. 

In certain cases it becomes necessary to carry high-tension 



LINE CONSTRUCTION. 



577 



conductors underground. This is always to be avoided if 
possible, but if necessary it can be done at present up to pres- 
sures of about 25,000 volts. The best plan is to use a three- 
conductor lead-covered cable run in vitrified clay ducts, well 
drained, and with frequent manholes of ample size so that the 
cable joints can be carefully made and supported. 

Fig. 286 gives a full size section of the class of cable most 
commonly used for high-tension underground work. The 
insulation is of paper, well impregnated with insulating com- 
pound. Each conductor is served with a heavy coating of this, 
the interstices are packed with jute and insulating compound, 




25Q00 Volts 

Fig. 286. 

and the whole is given an external wrapping and then leaded. 
This insulation is wholly dependent on the integrity of the lead 
covering, and hence the joints must be made and protected 
most carefully, but it has proved very reliable. Cables are 
used mostly below 12,000 volts, for which the insulation need 
not be as thick as that shown. To a certain extent leaded 
cables insulated with rubber and with varnished cambric are 
also used. 

The jimctions between cables and overhead lines are danger 
spots with reference to lightning and static surges generally, 
and should be protected by static dischargers and lightning 



578 ELECTRIC TRANSMISSION OF POWER. 

arresters on just the same plan as high- voltage apparatus in the 
stations. Underground cables should be laid in duplicate, 
since a fault cannot generally be quickly found and repaired.* 

A well-nigh indispensable accessory of every power trans- 
mission line is a private telephone line connecting the power 
house with the sub-stations and with intermediate points. 
Such a line is usually carried on side brackets attached to the 
poles six or eight feet below the power wires. This line is most 
often a metallic circuit of galvanized iron wire, about No. 12 
in size or larger on long lines, carried on ordinary glass insu- 
lators and transposed every twenty poles or so. Such lines 
can be made to give fair service, but the transposition of the 
wires has to be very carefully adjusted to suppress induction. 
The lengths of wire under induction must agree, not within a 
few poles merely, but within a few feet, to avoid annoying sing- 
ing. The two sets of insulators should be kept at a uniform 
distance from the main line, and the wires should be drawn 
uniformly tight and so transposed that taking the line from end 
to end, each wire shall have just half its length on the upper 
and half on the lower bracket, or on the right and left insu- 
lators if a short cross arm is used. 

The wires, too, must be kept clear of grounds from foliage 
and other interference, in order to keep the inductive balance 
perfect. With care in stringing, the line can easily be kept in 
good operative condition, but is seldom free from some residual 
induction. Such lines should be fused, protected with light- 
ning arresters, and provided with insulated platforms for those 
using the instruments. 

A far better although considerably more expensive line is 
obtained by using the twin-wire insulated cable made for tele- 
phonic purposes. 

On long lines it is good policy to make provision, say every 
10 miles or so, for getting at the high-voltage line for repairs. 

* For much valuable though sometimes discordant information on 
modern line construction, see the Trans. Int. Elec. Congress, St. Louis, 1904, 
Section D, Vol. II, especially the papers of Baum, Gerry, Converse, Buck, 
Black well, and Nunn, to whom the author stands indebted. It will be 
sufficiently evident that the problems encountered are complicated and 
difficult. 



LINE CONSTRUCTION. 



579 



If the line is in duplicate, it should be so arranged that at these 
junctions jumpers can easily be put on or switches closed 
between wires in the same phase, and a section of one of the lines 
cut loose so that it can be readily handled. At such points 
there should be opportunity for cutting in a portable instru- 
ment on the telephone line. Telephone boxes, Fig. 287, much 
like, the ordinary police signal box, can be obtained, and may 
advantageously be permanently installed at the ends of these 




Fig. 287. 

line sections. These are good points, too, for installing line 
lightning arresters and making provisions for testing. 

The commonest accidents on high-voltage lines are short 
circuits from branches of trees and broken insulators. The 
effect of the first is to start an arc that is likely to burn down 
the line, if the branch is more than a mere twig. There are 
great fluctuations of current and voltage, and the character of 
the accident is generally evident. Broken insulators may in 



580 ELECTRIC TRANSMISSION OF POWER. 

dry weather produce no sensible effect at all, but if the cross 
arms are damp there may be serious leakage between line and 
line that sometimes ends by burning up the cross arm or even 
the pole top. Broken insulators can be replaced if necessary, 
while the line is ''alive," even when carrying pressures as high 
as 15,000 or even 20,000 volts. The line affected can be pulled 
or pushed clear of the cross arm and held clear while the line- 
man puts on a new insulator, preferably one with a top groove 
for ease of manipulation. Then the line can be pulled back 
into position and an insulated tie wire put in place, if needful, 
with long rubber-handled pliers. It takes a skilful and cau- 
tious lineman to do the job, but it can be done if necessary. It 
is best not to trust to rubber gloves, as they are seldom in good 
condition, and there is nearly always enough leakage around 
the pole top to give a powerful shock. Sometimes, when work- 
ing at such a job, a nail is driven into the pole well below the 
workman, and a temporary jumper thrown from it over the wire 
under repair so that the lineman will be less likely to get leak- 
age shocks, or the cross arm is temporarily groimded by a wire 
for the same purpose. 

Duplicate lines are much easier to repair, since one can then 
work on dead wires, and for very high voltages duplicate pole 
lines are better still; but, with care, it is far safer and easier 
to work on high- voltage lines than is generally supposed. 



CHAPTER XV. 

METHODS OF DISTRIBUTION. 

In most cases of power transmission, the primary object is 
the supply of power and Ught in various proportions through- 
out a more or less extended region. Therefore, the question 
of methods of distributing electric energy, after it has been 
received from the transmission line, must often be carefully 
considered. The subject may conveniently be treated in three 
divisions: First, distribution direct from the transmission cir- 
cuit without the use of special reducing transformers or sub- 
stations. Second, distribution from scattered sub-stations. 
Third, distribution from a main reducing station. These divi- 
sions do not have rigid boundaries and often overlap, but they 
involve three quite diverse sets of conditions. 

Into the class first mentioned fall all the ordinary electrical 
installations wherein the power station is separated from its 
load by a transmission line. This line is usually of moderate 
length, for otherwise the voltage used would need to be reduced 
for the working circuit, and the region supplied is generally a 
towTi or city of moderate size. Such cases are common enough, 
and generally arise from the existence of a convenient water- 
power half a dozen miles, more or less, from a town that needs 
light and power, or that has already a central station which 
from motives of economy it is desirable to operate by water- 
power. The power is therefore developed and new distribution 
lines are erected, or the old ones reorganized. The whole con- 
dition of things is closely similar to ordinary central station 
practice, save that the load is all at a considerable distance 
from the station. Only the use of alternating current need be 
considered, since this current alone is practically employed for 
general purposes at distances above' a mile or two. 

The rudimentary map, Fig. 288, gives a case typical of many. 
The power station is at A, with a line across country to the 
town which is to be supplied with light and power. The dis- 

681 



582 



ELECTRIC TRANSMISSION OF POWER. 



tance to the town, A B, is perhaps four miles. Now the prob- 
lem is to distribute the energy derived from A over the town 
in the best and most economical way. Since much lighting as 
well as motor service is to be done, good regulation is essential, 
while abundance of water makes small variations in efficiency 
of little moment. The town is scattered, with a main business 
street C, running lengthwise through it. 



BUNKVlLLC 





Fig. 288. 



There is here little object in a sub-station, for the distances 
are too great for convenient distribution at low voltage, and 
the short transmission makes it desirable to avoid raising and 
reducing transformers. The choice of a system is the first con- 
sideration. This is not a question of such vital importance as 
the average salesman hastens to proclaim. The skilful organi- 
zation of the installation will make much more difference in 
the general success of the plant, than the particular species of 
apparatus used. This should, however, be determined with 
due regard to the local conditions. 

Any alternating system except plain monophase can be con- 
veniently used, and monophase is inapplicable only in default 
of suitable motors, which are not at the present time available 
in this country, at least in any form which warrants their use 
in cases where motors are to form any considerable portion of 
the total load. With a moderate amount of motor service in 
small units, the monophase system answers the purpose excel- 
lently. Something depends on the character and amount of 
the motor service. If it be very considerable and in all sorts 
of service, general experience both in this country and abroad 
indicates some advantages in triphase apparatus. This advan- 
tage, however, depends more on the ease and economy with 
which a triphase distribution can be carried out, when motors 



METHODS OF DISTRIBUTION. 583 

and lights are to be served in the same territory, than on any 
intrinsic advantages in the motors. When made with equal 
care and skill, all polyphase motors are substantially alike in 
their properties. Details of the various systems of distribu- 
tion will be given in treating sub-station work. Where the 
motor service consists of a few large units, even the monophase 
system with synchronous motors is entirely practicable, 
although seldom advisable. Diphase and triphase systems can 
be advantageously applied to any case that is likely to arise, 
and which one will best fit it is a matter that only a trained 
engineer with full knowledge of the local conditions can prop- 
erly decide. 

Of far more importance are the general methods employed in 
carrying out the electrical distribution, and these are applicable 
with almost equal force to any sort of alternating system. 

First in importance is the maintena.nce of a uniform voltage 
on the primary service lines. This voltage should, as far as 
possible, be the same at every transformer and should be con- 
stant, save as it may be raised to compensate for the loss in the 
secondaries. 

The first step toward obtaining this uniformity is to assume 
a fictitious centre of distribution as at D, Fig. 288. This should 
be chosen at or near the centre of load, generally in the business 
centre of the city. If the office of the operating company is 
conveniently situated, it should be used as a habitation for the 
centre of distribution, at which supplies can be kept and meas- 
urements made. D is taken as the termination of the trans- 
mission line proper, and acts in the capacity of a central station 
toward the primary service wires. As a preliminary toward a 
more exact regulation, there must be means for keeping the 
voltage at this point D up to the normal under all conditions of 
load. The most obvious suggestion is overcompounding the 
generators for constant voltage at D, and this is often advisable, 
though it must be remembered that compound winding is by 
no means the only and not always the best means of securing 
constant voltage at a point distant from the generator. 

When the circuit is nearly non-inductive, and the current 
therefore very nearly in phase with the E. M. F., or when the 
power factor can be kept very nearly constant, compounding 



584 ELECTRIC TRANSMISSION OF POWER. 

works admirably, and so is readily applicable to cases where 
lighting is the main work to be done, or where synchronous 
motors keep up the power factor of the system. 

If, however, the load is largely of induction motors, running 
at all sorts of loads, or is otherwise of strongly inductive char- 
acter, compound winding alone will not suffice to keep constant 
voltage at the point D. It will fail in proportion to the amount 
of compounding necessary to be employed, and for two reasons: 
first, because of the direct effect of the lagging current on the 
excitation necessary; and second, because, as has already been 
pointed out in Chapter VI, the lagging in phase of the current 
disturbs the fimctions of the commutator. It is sometimes 
desirable to bring ^'pressure wires" back from D to show at the 
station exactly the condition of things at the load, so that the 
voltage may be maintained by hand regulation, if necessary. 
This is, of course, a temporary expedient with a compound- 
wound machine, but it may avert frequent bad service. The 
pressure wires may come either from the primary circuit at the 
centre of distribution, or from some point of the secondary 
system which is chosen to represent average conditions of load. 
The latter is the preferable method, if there is a fairly complete 
system of secondary mains. The pressure wires may be taken 
as a guide for close hand regulation, or may operate some form 
of automatic control of the field rheostat. Neither hand nor 
automatic control is very satisfactory, if the generator requires 
great change of excitation mider change of load. For the class 
of power transmission under consideration, it is therefore better 
to use a generator of moderate inductance and armature re- 
action, whether it be compounded or otherwise regulated. 

For the pressure wires may be substituted a compensated 
voltmeter, arranged to take account of the drop in the line 
and show at the station the real voltage at the centre of dis- 
tribution, provided the power factor does not change so errati- 
cally as to vitiate the compensation. 

Granted now that means are taken to regulate the voltage 
at D as it would be regulated if the generator were at that 
point, the distribution problem is the same as that in an ordi- 
nary central station. Most alternating stations, however, are 
far from well "organized in this respect. Nothing is at present 



METHODS OF DISTRIBUTION. 



585 



commoner than to find an alternating station which receives 
pay for not more than one-half of the energy delivered to the 
lines, and sometimes this low figure falls to one-third or even 
a quarter. This unhappy state of things is due mainly to 
badly planned secondary circuits and to the indiscriminate 
use and abuse of small transformers. The alternating current 
transformer is a marvellously efficient and trustworthy piece 
of apparatus, and, perhaps in part for this very reason, it has 
been often the victim of wholesale misuse. Without going in 
detail into the case of sub-station vs. house-to-house distribu- 
tion, it is sufficient to say that the essential thing for efficiency 
is to keep the transformers in use well loaded and hence at 



^^^ 




n 



1 



Fig. 289. 



their best efficiency, and that for this purpose a few large trans- 
formers are, on the whole, much better than many small ones. 
The reason for this may be best shown by taking the following 
practical example: 

A given region requires, let us say, 250 incandescent lamps 
or thereabouts, together with fan motors and perhaps an occa- 
sional large motor. These are distributed among a score of 
customers scattered over a couple of blocks, Fig. 289. The 
blocks are, say, 200 ft. long, with alleys cutting them in two. 
Now these customers may be supplied from individual trans- 
formers, or all may be supplied from one transformer. In 
either case the lines should be carried in the alley. In the 



586 



ELECTRIC TRANSMISSION OF POWER. 



former case 20 transformers would be connected to service 
wires attached to the primary service main a b. These trans- 
formers would average, say, 12 lights capacity each (600 watts). 
In the latter case, a b would be a secondary main supplied from 
a single transformer of 12,000 watts capacity. Now, assuming 
a load such as would be met in ordinary practice, let us examine 
the transfor merlosses in each case. The day may conven- 
iently be divided into three periods in considering load : 7 a.m. 
to 5 P.M. forms the day load of motors and a few lights; 5 p.m. 
to 12 night, the evening load; and 12 to 7 a.m., the morning 
load. During the first period we may assume 15 transformers 
to be quite unloaded, 2 to be three-quarters loaded on motor 



— 










■■ 




o 














CO 


/ 


/ 










o 


/ 












u> 


/ 












o 


1 












•«t 














o 














CVI 


























' 



2 00 3 00 

Fig. 290. 



400 



500 



6 0O 



work except during the noon hour, and 3 transformers to be 
one-quarter loaded on day lights. 

During the second period we will assume the motors to be 
off, 8 transformers to be three-quarters loaded on the average 
from 5 imtil 7 p.m., and the rest one-quarter loaded from 5 until 
midnight. 

For the third period, it is safe to assume 15 transformers to 
be unloaded and the other 5 one-sixth loaded from midnight 
until 7. 

Now the efficiency curve of a 500 or 600 watt transformer at 
various loads is approximately as shown in Fig. 290, derived 
from a consideration of several transformers of different makes. 



METHODS OF DISTRIBUTION. 687 

The constant loss, when the transformer is run unloaded, is 
about 30 watts. 

On the above assumptions, and knowing the efficiency of the 
transformer at various loads, it is easy to calculate for each 
period the total energy supplied and the transformer output 
which is delivered and paid for. The result of this calculation 
is as follows: 

1st Period. 2d Period. 3d Period. Total. 
Energy Supplied, Watt Hours, 18,480 20,420 10,060 48,950 

Energy Delivered, " " 10,450 17,700 3,500 31,650 

Therefore, barely six-tenths of the energ}^ supplied to the 
transformers is delivered by them to the consumers. And this 
is a condition of thhigs more favorable than is usually found in 
stations of moderate size, using, as many of them do, small 
transformers. 

The other method of distribution is to use a single large trans- 
former in place of the small ones, and distribute to all the dis- 
trict by secondary mains. 

Now the efficiency of a 10-12 KW transformer is very closely 
that shown in Fig. 291. Moreover, the energy consumed when 
running without load is hardly more than 150 watts, so that 
the transformer, when absolutely unloaded, wastes only one- 
fifth of the energy wasted by the small transformers of the same 
total capacity. Taking the output for the same periods as 
before, a much better result is reached, as follows: 

1st Period. 2d Period. 3d Period. Total. 
Energy Supplied, Watt Hours, 11,800 19,660 5,880 37,290 

Energy Delivered, " " 10,450 77,700 3,500 31,650 

With the single large transformer, more than 80 per cent of 
the energy supplied to it is delivered on the customers' circuits. 
This means that for a given amount of energy supplied from 
the station, one-third more revenue will be obtained if the dis- 
tribution be accomplished by a large transformer as against 
quite small ones. Such a difference is important, even in a 
plant driven by cheap water-power. Besides, for a given 
amount of energy delivered to the customers, high-plant effi- 
ciency means, smaller first cost of plant. With distribution 
by secondary mains, not only will smaller dynamos at the 
power station suffice for the work, but the cost of the trans- 



588 



ELECTRIC TRANSMISSION OF POWER. 



former capacity necessary is enormously reduced. ' In the 
house-to-house distribution it is quite possible for any trans- 
former to be loaded with all the lights connected to it. When 
twenty customers are supplied from a single transformer, the 
chance of such an occurrence is almost nil. In the hypotheti- 
cal case just discussed, certain of the transformers would be 
called on for full output almost daily, while all of them would 
be subject to such a demand. The largest total regular out- 



<i^ 










^^^ 
























o 




/ 


^ 




















o> 




/ 






















o 


/ 
























o 

2 
LJ 

o 




















































O 


























CO 


























o 



























4 5 6 
Fig. 291. 



10 



|i IE 



put, however, would be not much over one-half the aggregate 
transformer capacity. So, instead of using a 12 KW trans- 
former to replace 20 small ones, in reality a smaller one, say 
one of 10 KW, would be ample. 

In point of cost, the single transformer would have the ad- 
vantage by not less than $250, enough in most cases to pay for 
the difference in secondary wiring. In regulation, too, the 
single transformer has the advantage, for the load is less liable 



METHODS OF DISTRIBUTION. 689 

to sudden fluctuations, and the transformer itself regulates 
more closely. 

In practice it is best to go a step further than shown in 
Fig. 289, and connect the secondary mains at a and b to the next 
section of secondary just across the street, and also c with the 
main in the next alley, so as to form, at least in the region of 
dense load, a complete secondary network. Thus, each trans- 
former can help out its neighbor, in case of need. The second- 
ary mains should, in so far as is practicable, be designed for 
the same loss of voltage, and the compounding and other regu- 
lation applied to the generator should be arranged to compen- 
sate for the loss of voltage in the transformers, and to hold the 
voltage as steady as possible in their secondary mains. The 
perfection of such regulating arrangements depends, of course, 
on the uniformity of the distribution of load; but with a little 
tact in arranging the circuits, variations in voltage at the lamps 
can often be kept within 2 per cent of the normal pressure. 
In large systems, as will be presently shown, evQn better work 
can be done. 

An essential point m the use of secondary mains is the em- 
ployment of fairly high voltage. The general law, that the 
amount of copper necessaiy in a given distribution varies 
inversely as the square of the voltage, applies here with great 
force. Not less than 110 volts should be used, and a pressure 
of 115 to 120 volts is better, as it gives equally good service 
with a quarter less weight of copper. From the present outlook 
even higher voltage is becoming practicable. 

It is not always advisable to do all the work of distribution 
by secondary mains. In districts where the service is scat- 
tered, a few small transformers of various sizes can be very 
advantageously used, but should be generally employed as 
a temporary expedient only, and shifted to another field of 
usefulness when the service grows heavy enough or stable 
enough to justify installing secondary mains. 

Recurring now to Fig. 288, we have found that the best pro- 
cedure is to use an alternatmg system, compoimded or other- 
wise regulated so as to hold the voltage as nearly as possible 
constant at the secondary terminals of the transformers. 



590 ELECTRIC TRANSMISSION OF POWER. 

These should be large enough to do all the work within a dis- 
tance of 200 ft., more or less, and should feed secondary mains 
at a pressure of, say, 115 volts. When these mains are more 
than usually long, it is best not to feed current directly into 
them, but to employ feeders connecting, for instance, c, Fig. 289, 
with points midway between c and a, and c and h, respectively. 
Neighboring secondaries may often be interconnected with 
great advantage. 

As to the primary distribution, we have assumed a centre 
at D, Fig. 288. From this point feeders should extend to 
primary mains connecting the transformers more or less com- 
pletely, preserving nearly equal drop in voltage from D to 
each transformer. The degree of elaboration in this primary 
network is a matter to be determined by local conditions. If, 
for example, the plant is of rather small size and the drop from 
B to C, Fig. 288, is not above 1 or 2 per cent, the transformers 
may be connected to short branch lines crossing B D C Sit vari- 
ous points, without any further complications, or the main 
line may be branched at B, each branch having short cross 
feeders, while with other distributions of load the primary 
lines may be quite completely netted, with regular feeders 
at D. 

The motor service may often require special treatment. It 
often happens that it is best to feed large single motors or 
groups of motors from special transformers, which will gener- 
ally be large enough to avoid the objections adduced against 
a general house-to-house transformer system. Such special 
transformers avoid throwing a large and varying load on 
the secondary lighting mains during the hours of '' lap-load" 
when it might be objectionable, and thereby avoid needlessly 
heavy mains and annoying variations of voltage. 

It must be remembered that Ohm's law is a very stubborn 
fact. Any apparatus that takes a large and variable current is 
liable to interfere with regulation. There is no such thing as 
a motor, either for continuous or alternating currents, which 
will not affect the lighting service. The nearest approach to 
such a motor is obtained by arranging the distributing system 
so that the largest current taken by the motor will be insuffi- 
cient noticeably to disturb the regulation of the lamps. 



METHODS OF DISTRIBUTION. 591 

This means that care should be taken, in arranging the dis- 
tribution, to avoid overloading the lighting mains with motors. 
It is an easy matter to determine the effect of the motor cur- 
rent by calculation if the current is continuous, and by experi- 
ment or calculation for alternating current. In the latter case 
the easiest way is to connect the motor with any convenient 
main and put on load with a brake — even a plank held against 
the pulley will do. Put an ammeter in circuit, and if at the 
rated amperage of the motor the fall in volts at the transformer 
is enough to endanger regulation, the motor should be put on 
transformers of its own. Generall}^ the likelihood of trouble 
can be judged from the size of the motor and the load on the 
mains, without experiment. 

One of the nice questions to be decided, in such a plant as is 
under discussion, is arc lighting. The most obvious method 
of arc lighting from a transmission plant is to use alternating 
motors to drive arc d5riiamos, either belted or directly coupled. 
This method is in use in a good many plants, and works ad- 
mirably, although the efficiency is not all that could be desired, 
being probably about 70 per cent at full load, reckoning from 
the energy received by the motor to that delivered at the lamps 
under the most favorable commercial conditions. That is, 
for the operation of each 450 watt (nominal 2,000 c. p.) con- 
tinuous current arc, at least 650 watts would have to be deliv- 
ered to the motor. In working commercial circuits, on which 
the number of lights varies greatly, the efficiency at light loads 
would be greatly reduced, and might easily fall to between 50 
and 60 per cent. 

This is not a cheerful showing, and much ingenuity has been 
spent in attempting to remedy such a state of things. For 
street lighting, the scheme is reasonably good, but it breaks 
down in commercial lighting. 

In cases where plants operate low-tension direct current 
systems via motor-generators or rotary converters, the solu- 
tion of the difficulty is simple, since all the commercial arcs 
can readily be worked at constant potential, using preferably 
enclosed arc lamps taking from 5 to 7 amperes. The effi- 
ciency of the rotaries is high and the loss in the mauis is not 
great, so that by this means the only circuits that need be 
worked on the series system are the street lights, which form 



592 



ELECTRIC TRANSMISSION OF POWER. 



a nearly constant full load. When the distributing system is 
alternating, one can still use constant potential lamps for the 
commercial circuits with fairly good results. 

Alternating constant potential enclosed arc lamps have at 
the present time been brought to a state that justifies their 
extensive use, and yet it must be admitted that they are some- 
what less satisfactory than the direct current arcs. Taking 
lamps as they are found commercially, and comparing direct 
current with alternating current enclosed constant potential 
arc lamps, the following results were obtained by a committee 
of the National Electric Light Association appointed to deal 
with arc photometry: 





■U3 

U 

o 


3i 


Mean 
Spherical C. P. 


Watts 
Per M. S. C. P. 






Opal 
Globe. 


Clear 
Globe. 


Opal 
Globe. 


Clear 
Globe. 




Direct 


4.90 


529 


155 


182 


3.41 


2.90 


Average of 
8 lamps. 


Alternating . . 


6.29 


417 


114 


140 


3.66 


2.98 


Average of 
7 lamps. 



These figures show that even considering the energy abso- 
lutely wasted in dead resistance in the direct current constant 
potential lamp, it still has a slight advantage in efficiency over 
the alternating lamp, not enough, however, to compensate in 
addition for the loss of energy incurred in changing from 
alternating to direct current. 

The alternating lamp is at a slight further disadvantage in 
that it requires rather the more careful handling, and a rather 
better grade of carbons. It also is liable to give trouble from 
noise, although the best recent lamps are comparatively free 
from this defect, which has in the past been a serious objec- 
tion to this type of lamp. Some noisy lamps are still to be 
found on the market, and a hard carbon will make almost any 
lamp sing. Most of the noise originates in the arc itself, and 
it is considerably reduced by enclosing the arc and using a 
non-resonant gasket on the outer globe. Good lamps care- 
fully operated are capable of giving very excellent service, 
and are entirely adequate for commercial circuits under ordi- 
nary circumstances. 



. METHODS OF DISTRIBUTION. 593 

It would appear at the first glance at the table just given, that 
enclosed arcs of either type are little, if at all, more efficient 
than good incandescent lamps. 

The difference between them is in fact not very great, but 
incandescent lamps are generally rated on horizontal candle 
power and on their initial, not their average, efficiency. With 
due allowance for this, the efficiency of the best commercial 
incandescent lamps per mean spherical candle power ranges 
from 4 to 4.5 watts per candle power, so that the arcs have 
still a fair margin of advantage, increased by the better color 
of their light. In using alternating arcs for commercial work, 
their performance is much improved by pushing the current up 
to about 7.5 amperes, at which point the lamp is more efficient, 
more powerful, and, if properly adjusted, steadier. 

In street lighting, the best results have so far been obtained 
by operating alternating arcs in series on the constant current 
system. This requires special lamp mechanism and special 
devices for changing the constant potential alternating current 
of the transmission line to a constant alternating current of 
voltage automatically varying with the requirements of the 
circuit. 

Such constant current regulators vary considerably in detail, 
but the underlying prmciple of all of them is as follows: A 
heavy laminated iron core is surrounded by a movable, counter- 
balanced coil, through which the current to be regulated flows. 
Any change in the position of this coil changes the reactance 
of the combination, and h^ice varies the current. The coil 
is counterbalanced, until, when the normal current is flowing, 
the coil floats in equilibrium, the opposing forces being gravity 
and the attraction or repulsion between the coil and the mag- 
netized core. Then any change of current due to varying con- 
ditions in the external circuit establishes a new position of 
equilibrium for the coil in which the changed reactance brings 
the current back to its normal amount. Sometimes the regu- 
lator is combined with a transformer which receives current 
from the transmission system at any convenient voltage, while 
the floating coil acts as a secondary and delivers constant 
current to the lighting circuits. 

This is the arrangement of the constant current transformer 



594 ELECTRIC TRANSMISSION OF POWER, 

system as operated by the General Electric Company. Fig. 1, 
Plate XXIII, shows the internal arrangements of the trans- 
former. It has two fixed primary coils at the top and bottom 
of the structure, receiving current from the main line, and two 
floating secondary coils counterbalanced against the repulsion 
of the primaries, and balanced against each other by the double 
system of rocker arms visible at the top of the cut, which are 
supported on knife edges. The short balance lever in the fore- 
ground is attached to the rocker arms by a chain, and is like- 
wise pivoted on knife edges, while it carries on its longer sector, 
suspended by a chain, the adjustable counterbalance weights. 
The current can be adjusted at will within reasonable Hmits 
merely by adding or taking off counterbalance weights. The 
whole apparatus is enclosed in a deeply fluted cylindrical cast- 
iron case filled with paraffin oil, which serves the double pur- 
pose of giving high insulation and also damping the oscillations 
of the floating coils. Hence, the system has been jocularly 
dubbed the 'Hub" system, and the name has every appearance 
of sticking. Transformers of this type are built for as large a 
load as 100 series arc lights, in which instance they are usually 
arranged on the multi-circuit plan, with two 50-light circuits. 
Some of these big tubs have four primaries and four second- 
aries, while the smallest sizes have but one of each. 

The diagram. Fig. 292, gives a very clear idea of the circuits 
of this simpler form of the apparatus, and of the shifting of the 
secondary coil under changing load. The secondary is shifted 
by hand into the short-circuit position and a plug short-cir- 
cuiting switch inserted just prior to starting up, and when the 
primary current is on, the short-circuiting switch is withdrawn, 
throwing the current upon the lamps. 

These transformers regulate promptly and steadily, and hold 
the current quite accurately constant from full load down to 
as low as one-quarter load. Their efficiency as transformers 
of energy is high in spite of a rather unfavorable form of the 
magnetic circuit, being 95-96 per cent at full load in the aver- 
age sizes. From the nature of the case, however, the power 
factor of such apparatus is not as high as would be desirable, 
being about 80 per cent at full load, and falling off in practically 




Fig. 1. 




Fig. 2. 



PLATE XXTII. 



METHODS OF DISTRIBUTION, 



595 



Sscondsry 
5.RSwit.ch. , ^x ^X— 



^ 1 SecQT-idefy 



UT-si 



t. ( Sjconds 



"^ 



T 

-X— ' 



conda •y_ ^<|- 



^ q-:^ 



innars^. 



rj^D.PSwftcK 
J/Jr-ruse Box . 

ft 



Anc Lan-ip. 
-Shor-t, Circuit. 
-Half Uoad 

-ruii 



main Line Primary. 



Fig. 292. 



linear proportion as the load decreases. For this reason the 
apparatus, when put into action, throws a nasty inductive load 
upon the system, and it is good policy to cut it in with a water 
rheostat in the primary circuit so that the load may go on 
gradually. An ordinary barrel nearly filled with pure spring 
water, with a fixed electrode at the bottom and a movable one 
at the top, each a little smaller than the barrel head, makes 
a very efficient rheostat for an ordinary circuit of 2,000 volts 
or so. 

Owing to the low-power factor, such apparatus should not 
be used on circuits likely to be worked much at partial load. 
It is very well suited, however, to street lighting, and has come 
into very extensive use. 

A similar device has been considerably used in the practice 
of the Westinghouse Company, differing, however, in that the 
transformer and regulator functions are not combined. The 
regulator is shown in Fig. 2, Plate XXIII, and in virtue of what 
has already been said the cut is self-explanatorj^ The balance 
floating coil inserts automatically the reactance necessary to 
hold the current constant. Regulators are made for a range 
of action varying from 25 per cent to 100 per cent of the whole 
load, according to the requirements of the case, and, of course, 
are of greater size and cost according to the range required. 
They hold the current closely at a uniform value, and have, 



596 ELECTRIC TRANSMISSION OF POWER, 

probably, at full load a somewhat better power factor than the 
combined apparatus just described. They have the same 
rapid falling off of power factor at low loads, however, and 
must be used in connection with a separate static transformer 
to give the required voltage on the arc circuit. They could, of 
course, be installed directly on the distribution circuit, but at 
the risk of a ground on the arc circuit involving the whole sys- 
tem in trouble, so that practically they are regularly used with 
transformers. The efficiency of this system does not differ 
materially from that of the tub system already discussed, and 
the operative qualities of both are much the same. 

The ordinary series alternatmg arc takes about 6.6 amperes 
and 425 watts, and at^this input is materially better as an illum- 
inant, light for light, than the so-called 1,200 c. p. open arc or 
the enclosed continuous current arc taking 5 amperes or there- 
abouts. To compete on favorable terms with the 9.5 ampere open 
arcs, or the 6.6 ampere enclosed continuous current arcs, the 
current in the alternating system should be carried up to at 
least 7.5 amperes, at which point it is substantially the equal of 
the others in practical street lighting. 

Used thus on the street, the slight noise of the alternating 
arc is not noticeable, and experience has shown the operation 
of the system to be eminently satisfactory. For commercial 
lights the series alternating arcs are not to be commended, 
since the power factor of the regulating devices is objection- 
ably low at partial loads. The choice for such work lies be- 
tween conversion to continuous current and the use of con- 
stant potential alternating arcs, with the advantage, at the 
present time, rather in favor of the former expedient. Re- 
cently some very good results have been obtained in arc light- 
ing by means of the mercury rectifier already described. The 
best success has been had with the so-called " luminous " arc. 
In this the lower electrode is chiefly of compressed magnetite in 
a sheet iron tube, and the upper of copper. The light is given 
mainly by the arc itself, is brilliant and highly efficient, and is 
thrown well out near the horizontal. The ordinary street arc of 
this type takes about 4 amperes and 70 volts. 

Eor much commercial work there is no need of using arcs at 




/u^uaE 



LAMP PfTT/COAT 
6L0B£ H0LD/N6 SCPSIvJ 



.HOLDER POPC£iAIN 




HeATC/} POfiCeiAWl ^ 
HiATtR TUB£\ 3; 

6Lowea} 



Fig. 2. 



PLATE XXIV. 



METHODS OF DISTRIBUTION. 697 

all, and often incandescent lamps may replace arcs to advan- 
tage. In cases where fairly powerful radiants are necessary, 
and particularly where the color of the light is important, very 
good results can be obtanied by the use of Nernst lamps. 

The Nernst lamp is a modified incandescent in which the 
light-giving body is not a filament in vacuo, but a stick of re- 
fractory material driven to high incandescence in air. The 
material used is akin to that used in Welsbach gas mantles, 
mainly thorium oxide. A non-conductor when cold, it must 
be artificially heated to start the current, when it becomes a 
tolerable conductor and can be successfully worked at a higher 
temperature, and hence a higher efficiency, than an ordinary 
incandescent lamp. The principle involved is simple, but the 
accessory parts needed produce a lamp which, while less com- 
plicated than an arc lamp, requires more attention than an 
ordinary incandescent. 

The Nernst lamp as used in practice is shown in Plate XXIV, 
of which the upper figure shows the connections of the 3-glower 
lamp which is the ordinary form, and the lower, the 3-glower 
lamp complete. The essential parts are the glower, the heater, 
the heater cut out, and the ballast. 

The first named is a stick, of oxides, in a 220-volt lamp about 
an inch long and ^V inch in diameter, into the ends of which 
are fused tiny platinum balls connected to the lead wires. 
The heater is a spiral of fine platinum wire embedded in a tube 
of refractory enamel, and the magnetic cut-out merely serves 
to prevent this from remaining m circuit after the current is 
fairly established through the glower. The ballast is a resis- 
tance of fine iron wire in series with the glower, and enclosed 
in an oxygen free tube to prevent oxidation. As iron rapidly 
increases in resistance when heated it prevents too large a cur- 
rent through the glower, which decreases its resistance when 
hot, in case of a rise in voltage, and thus tends to steady the 
action of the glower. The cuts speak for themselves. 

The lamps suffer seriously from a species of electrolytic 
action on the glower when used on direct current, and thus 
are essentially alternately current lamps doing well at periodi- 
cities from 25^ up, with rather better life at the higher fre- 
quencies. The life of the glowers is 600 to 800 hours, some- 



598 ELECTRIC TRANSMISSION OF POWER. 

times more, and the mean spherical efficiency when used with 
a light-diffiishig globe, as is usually necessary on account of 
the high intrinsic brilliancy, is slightly better than that of an 
a.c. arc lamp. The Nernst lamp, however, is arranged inten- 
tionally for a strong downward distribution of light, and hence 
in many situations does considerably better, watt for watt, 
than the enclosed arcs. Figures on the maintenance of these 
Nernst lamps vary widely, but the best information at hand 
indicates that it is relatively rather less than for arcs. The 
color of the light is almost pure white, very conspicuously 
better than that of any form of enclosed arc, and the illumina- 
tion is beautifully steady. The lamps start rather slowly, 
rising to full brilliancy in forty seconds or so. They have 
come into considerable use already, and for commercial work 
on alternating current transmission systems have much to 
recommend them. 

Very recently highly efficient incandescents with metallic fila- 
ments have been introduced. The best known of these are the 
tantalum and the tungsten lamps, the former requiring 2 watts 
per m. h. c. p., the latter about 1.25 watts. The tantalum lamp 
gives poor life on a. c. and the tungsten filament is as yet ex- 
tremely fragile and so plastic when hot that the lamp must be 
burned tip do^vnward. But improvements in metallic filaments 
are progressing rapidly, and there is a strong probability that 
they will eventually displace arcs for many, if not most, pur- 
poses. They give much longer life than ordinary incandescents 
and considerably surpass in efficiency all ordinary forms of 
enclosed arcs. 

The gen.eral principles of distribution laid down hold what- 
ever alternating system is used. Polyphase and other modified 
alternating systems require special treatment in the details of 
distribution, but not in the broad methods employed. 

Motor service should generally be cultivated as a desirable 
source of profit and an excellent way of raising the plant effi- 
ciency. A motor load, if of numerous units or a few steadily 
loaded ones, is remarkably uniform. Fig. 293 shows the load 
line of a three-phase power transmission plant. The motor 
load consisted of about fifty induction motors of various makes, 
aggregating nearly 350 rated HP. The curve shows the prim- 



METHODS OF DISTRIBUTION. 



599 



ary amperes in one leg of the circuit throughout the twenty- 
four hours. It was taken on a day in early August when the 
lamp load was very light and reached its maximum as late as 
8 P.M. The motor load, save for the sharp decline during the 
noon hours, was very steady, although there were frequent 
variations through a range of a few amperes, too brief to appear 
on the diagram. In this case and at this season of the year 
there is no ''lap load." The distribution is, as far as possible, 
from secondary mains, and even in winter, when the lap load 



Ifl 


















o 






/ 




( 


1 






^ 










1 








o 


















or 


















o 










/ 






k 


q: 






/ 


\ 


/ 


\ 




\ 


< 

o 
















\ 



ia 



€AM 



M 

Fig. 293. 



6 PM 



ta 



is prominent, although the motors still require the major part 
of the output, the regulation of the system is admirable. 

Thus, even a heavy motor load gives very little trouble with 
a properly designed system of distribution and judicious hand- 
ling. The things to be feared are large motors running on very 
variable load, motors with bad power factors carried by over- 
loaded transformers, and overloaded conductors during the 
period of lap load. Now and then a system is installed for 
motor service only or with special motor circuits. In this case 



600 ELECTRIC TRANSMISSION OF POWER. 

it should be remembered that there is no need for any very- 
close uniformity of voltage throughout the system, and that to 
attempt it means waste of time and money. The circuits can 
be laid out with reference to the desired efficiency alone, for 
in most cases even 10 per cent variation in voltage between one 
motor and another is of little consequence. 

The distribution of power from scattered sub-stations fed 
by a common line, involves some of the most intricate and 
puzzling problems to be found in power transmission. Such 
distributions generally arise from an attempt to supply from a 
common power plant, energy for divers purposes to several 
separate towns or regions, having different requirements. In 
the main such plants require special treatment in order to 
secure decent service. A great variety of cases may arise, 
almost every plant having peculiarities of its own, but in 
general they will fall into one of the three following categories : 

1. Radial distribution from a centrally located station. 

2. Radiating distribution from an eccentric station. 

3. Linear distribution. 

1. The first-mentioned class consists of those plants which 
supply from a single station power to different localities lying 




Fig. 294. 

in different directions, and generally at different distances. 
Fig. 294 shows the character of the conditions thus met. A is 
a generating station, the position of which may be determined 
by various reasons — the existence of valuable water-power 
being the commonest; B, C, D, E, F, are the various points to 
be supplied with power. They may be at any distances, and 
of any sizes or natures. Usually the greatest distance involved 
will not be coupled with the greatest load, and the situation 
is otherwise inconvenient. If all the loads were large, the 



METHODS OF DISTRIBUTION. 601 

simplest procedure would be to install one or more generators 
for each circuit and operate them independently. Or if by 
good luck two or more load points were of similar size, distance, 
and character, they would naturally be operated as if they 
were one. 

To consider methods of operation more in detail, imagine a 
system consisting of the station A, a load at B consisting of 
150 KW in lights and motors, largely the latter, distant 3 
miles; and a load at E, 6 miles away, of 250 KW, mostly incan- 
descent lamps. At both B and E, it would be desirable to 
distribute at the voltage of transmission without a general 
reducing station. In such a plant it might be possible to 
operate B and E from separate generators, compounding 
them or using the regulating methods already described. But, 
if day lighting at E is to be attempted, it would be necessary 
either to rmi one dynamo all day at a trivial load, or to throw 
this day work in on the other circuit and take the voltage 
as it chanced to come. 

With the ordinary amount of loss in the line A B, the re- 
sult would be decidedly bad regulation at E, with only motors 
Sit B or E the case would be very simple; the station would be 
regulated with reference to the lighting load alone, but with 
lights at both places there must be good regulation at both. 
During the day at least it would be desirable to work both lines 
from the same generator. The first step in this direction 
would be to install at A a hand regulator to control the line 
A E. As already pointed out, a motor load is often fairly 
steady except at certain times, so that the regulator would 
require little attention save in the early morning and at noon. 
Before the motor load fell off in the afternoon, it would prob- 
ably be desirable to start a separate dynamo for E. 

In operating both lines regularly from the same generators, 
hand regulation on at least one of them would become neces- 
sary; on which one is a matter of relative convenience. If 
the distances A B, AE were much smaller, not more than two 
or three miles, it might be feasible to install both lines for 
small and equal drop — not over 2 per cent — so that, if the 
dynamo were compounded for an equal amount, the possible 
variations of voltage would be trifling. Such a plan cannot, 



602 ELECTRIC TRANSMISSION OF POWER. 

however, give really good regulation over any save very short 
distances without inordinate expense for copper. This sort of 
regulation by general average has been tried too often already, 
with disastrous results, and is quite out of place in serious 
power transmission work. 

li A E were ten or fifteen miles long the manner of opera- 
tion would become a still more troublesome question. Raising 
and reducing transformers would generally be used, and the 
best plan would probably be to install a pressure regulator in 
connection with the raising bank of transformers, and let the 
shorter line be taken care of by the compounding of the gen- 
erator or by a pressure regulator of its own. The latter pro- 
cedure is somewhat preferable. For if the drop in the lines be 
5 or 10 per cent and the loads variable, the work of regulation 
will be lessened by compounding the generator, if at all, for 
constant potential at its own terminals. The range of the 
hand regulation is thus lessened, since there is no attempt at 
over compounding; and two regulators requiring occasional 
adjustment are easier to handle than one which requires con- 
tinual juggling to produce indifferent results. 

In certain cases of heavy load there may be a regular sub- 
station at B or at E, the distribution at the other point being 
direct. Then the regulation question is better transferred to 
the sub-station, the generator being regulated for the loss in the 
other line, which as its load will usually be relatively small, 
should have a comparatively small drop. 

The most troublesome case that can arise is when power is 
to be furnished to a street railway at B or E, in addition to a 
general lighting and motor service. A railway load is so vio- 
lently variable that it cannot be operated in direct connection 
with an incandescent service unless this latter with the general 
motor load is so great as to quite dwarf the variations of rail- 
way load. Frequently, therefore, a separate generator should 
be devoted to the railway work. In case this cannot be done 
without great inconvenience, it may become necessary to 
install a sub-station at which the lighting circuits can be regu- 
lated either by hand or automatically. 

Suppose now that the problem is complicated by the addi- 
tion of loads at C, D, and F. These lines will be treated on 



METHODS OF DISTRIBUTION. 603 

the same general principles as the first two. To begin with, 
any line operating motors alone can be worked direct from 
the generator. Even if all the loads be mixed in character, 
two or more can often be found which through similarity of 
conditions can be worked together in parallel, either by a 
common regulator or by compounding the generator. The 
others should be treated as already indicated. At the worst, 
it might be necessary to install a regulator for each line. This 
is not really so burdensome as might be supposed, since several 
of the regulators will usually require infrequent attention, so 
that one man can manipulate the whole set. This line of action 
is similar to that followed in most large central stations, where 
feeder regulation, although rather a nuisance, is successfully 
accomplished without any particular difficulty. Feeder regu- 
lators for alternating circuits have, however, by no means 
received the attention that is their due. 

Pressure wires from each load point are desirable, though, 
if the load is such that the inductive drop is small or quite 
steady, the regulator can be as easily adjusted in accordance 
with the current on the line, or in accordance with the indica- 
tions of a compensating voltmeter. 

In the transmission and distribution of power from an eccen- 
tric station, the difficulties are many unless recourse be taken 
to a regulator sub-station. Fig. 295 shows a typical situation. 
Here A is the generating station and B, C, D, E, F, are the 
load points. If the distance from A to the nearest load is 



A- 




FlG. 295. 

great enough to require raising and reducing transformers, it 
is generally best to install a reducing sub-station worked like 
the central station A, Fig. 294. Sometimes, however, it is only 
half a dozen miles or so from A to the group of load points. 
The case is similar to that discussed in the first part of this 



604 ELECTRIC TRANSMISSION OF POWER. 

chapter, save that the load is m several distinct localities in- 
stead of being generally distributed. From this difference 
the complication arises. A certain proportion of cases can be 
treated readily, however, by choosing a point G near the centre 
of load and then running the lines G B, G C, G D, G E, G F with 
1 or 2 per cent loss wherever lights are to be furnished. Then 
by holding the voltage constant at G or slightly over compound- 
ing at that point, sufficiently good service can often be given. 
If the loads are very unequal in size, G may be chosen at or 
near the most important point and lines run to the others as 
before, with the regulation question confined practically to the 
first. If the load points are quite numerous and scattered, 




Fig. 296. 

Fig. 296 may be a preferable pla-n. Here two lines A B and 
A C are run and a group of load points is served from the ter- 
minal of each line. The groups shown are about equal, but 
sometimes it would be desirable to rim a separate line for a 
single point where the load was peculiarly heavy or trouble- 
some. 

These scattered distributions are fortunately mostly for 
motor service, so that regulation, in practice, is often easier 
than the situation indicates. They sometimes run naturally 
into the linear distribution, which, unless of trivial size, is a 
thorn in the flesh of the engineer. 

Fig. 297 is a type of this linear distribution, which is often 
met with in large transmission work and especially in long 
distance cases. 

The power station A is mainly intended to supply lights and 
power at B, which may generally be supposed to be the largest 
town in the immediate region. Incidentally it is highly desir- 



METHODS OF DISTRIBUTION. 605 

able to supply lights and power to C, Z), E, F, G, towns or 
manufacturing points at which electric power is needed. The 
main line A B may be taken as 20 miles, which is enough to 
disclose most of the difficulties. 

Of course, the line must be operated at high voltage with 
raising and reducing transformers. In nearly every case the 
latter would be placed in a regular sub-station, with appro- 
priate regulating apparatus for keeping uniform voltage 
throughout the primary and secondary networks in B. The 
loss of voltage in the line above may be assumed at 10 per 
cent, and the primary pressure at B as 20,000 volts. As B 

r 



Ll 



A> I — ' ' 1 ' V •» 

Fig. 297. 

comprises by far the largest and most important part of the 
load, attention should be first directed to complete regulation 
at that point. 

This can be best attained by first holding the primary pres- 
sure at B constant by compounding or other regulation at A, 
and second, by careful regulation of the primary and secondary 
feeders in the sub-station. In fact the whole transmission 
must first be treated with respect to results at B, while never- 
theless it is necessary to scatter power along the line at the 
points indicated. There may be present all sorts of require- 
ments. For example, at C there may be required 1,000 in- 
candescent lamps and a few motors; at D, 500 incandescents ; 
at E, a 50 HP motor and 300 incandescents ; at F, 300 incan- 
descents; and at G, 200 HP in motors and 200 lamps. 

Frequently the load at one or more points may consist of 
motors only. This case is not included above, since no special 
regulation is needed; the power has only to be transformed 
from the line voltage to that of the motors, neglecting the 
effect of varying loss in the line. 

Each of the cases noted involves the question of regulation 
in a somewhat troublesome form; at D, for example, the con- 
ditions imder which incandescent lamps must be supplied are 



606 ELECTRIC TRANSMISSION OF POWER. 

most severe. To begin with, at the nearest point of the main 
line, A B, the voltage may change by about 6 per cent, owing 
to varying loss in the line; the branch to D causes a trifle more 
variation, the drop in the transformers still more, and finally 
there must be added the loss in the secondaries up to the lamps. 
In all, these cumulative variations in voltage may be 10 per 
cent or more. At best, this means 5 per cent change of vol- 
tage above and below the normal. This is too great to allow 
what can be called good service, although worse is sometimes 
given. In fact such variation ought to be classified as out- 
rageously bad. To better matters, two methods are available. 

First, one may use a hand regulator in connection with the 
reducing transformers; for, in so large a system as that in- 
volved, the changes in voltage are relatively slow; and the con- 
ditions of load may be such that over compounding on the 
main line may partially compensate for the losses elsewhere. 
Or second, the lights may be operated by a dynamo driven by 
a synchronous motor. This procedure adds somewhat to the 
expense and trouble, but completely eliminates the loss in the 
line, since the speed of the motor is independent of the applied 
voltage, and incidentally, of the load. 

For small outputs a good induction motor serves the purpose 
well, for it is simpler to operate than the synchronous variety 
and can be made remarkably insensitive to changes of load 
and voltage. This motor generator device is an admirable 
resource when a very variable line voltage must be dealt with. 
In making the installation for a point like D, the actual varia- 
tion of the pressure at the point of tapping the main line should 
be ascertained, and the effect of the subsequent losses up to 
the lamps should be computed. If the resultant changes are 
frequent and considerable, a motor generator gives the best 
result. For gradual and moderate changes, an occasional 
touch at a regulator may be all that is needed, and now and 
then the resultant variation will prove to be not more than 2 
per cent above or below an assumed normal for the lamps, in 
which case the regulation often may take care of itself. 

At C there is a distribution equivalent to that from a small 
central station. The line pressure will generally have to be 
twice reduced before feeding the lamps. The choice of 



METHODS OF DISTRIBUTION. 607 

methods is the same as in the case just discussed, except that, 
with the losses of a double transformation and rather scattered 
service, regulation cannot be left to chance. Generally in a 
station of this size some regulation due to the distribution itself 
will have to be provided for, and the simplest course is to es- 
tablish a sub-station with one or more pressure regulators. 
This is operated just like the sub-station at B, being merely on 
a much smaller scale. A careful study of local conditions, 
however, is needful to enable one to discriminate between the 
two methods mentioned. 

At the station E the motor will take care of itself, but the 
lamps might give trouble owing to variations in motor load. 
If these are great and sudden, nothing save running from the 
motor a generator for the lights will answer, and even that will 
not be entirely satisfactory. If the load is steady and the 
lights regularly in use, as would be common in factory service, 
the loss in the branch line to E and the secondaries can be 
adjusted so that if the voltage at B is kept constant by regula- 
tion at A, that at E will be nearly so. This device is probably 
the one best suited to give good service at F. For G the same 
method holds, but with so large a proportion of motor load, 
separate transformers for the lights are almost necessary. In 
cases where there is no regulation at A for the loss in the line, 
pressure regulators or sometimes motor generators will have 
to be used at E, F, G. 

The various cases of linear distribution just considered are 
of necessity treated little in detail, since they are so much 
modified in practice by special circumstances. Enough has 
been said, however, to indicate the methods to be followed and 
to show how tactfully this class of problems must be treated. 

Finally comes that very important class of cases which 
involves the distribution of transmitted energy from large 
reducing stations. Such is the normal condition of affairs 
whenever power is transmitted to a city in large amounts for 
lighting and motor service. Passing over a few instances in 
which this power may be mainly utilized for driving by motors, 
or replacing by rotary transformers, existing central stations, 
one is confronted by the problem of constituting a great dis- 
tributing system for alternating currents; a system general 



608 ELECTRIC TRANSMISSION OF POWER. 

enough to be available for every service, and perfect enough 
to compare favorably with the great networks now worked by 
continuous currents. Until very recently this problem would 
have been insoluble in any practicable way, but to-day, thanks 
to the modern alternating systems and to the intelligent use 
and arrangement of large transformer units, it is possible sub- 
stantially to duplicate in convenience and efficiency the best 
direct current systems, while retaining the enormously valuable 
advantage of using high tension feeders. It must not be sup- 
posed, however, that the same procedure must suit both cases — 
the results but not necessarily the methods, must be in full 
accord. 

The basis of each system must be a carefully laid out network 
of working conductors, giving throughout the area of service 
a substantially uniform voltage as high as can conveniently be 
employed in the various receiving apparatus — lights, motors, 
and so forth. This voltage is practically determined by that 
of the incandescent lamps which are available. A few years 
ago 100 to 110 volts was the working limit of effective voltage 
between incandescent service wires (not of course the extreme 
voltage to be found between any two wires of the system). Of 
late the majority of important stations employ lamps of 115- 
120 volts. Now and then 120-130 volts is reached, and very 
recently there has been a strong movement toward boldly 
doubling the usual voltages and employing lamps made for 
200-250 volts. 

A considerable number of scattered small plants use such 
lamps, and in a few cases central stations have adopted them 
in connection with three-wire systems, using thus about 440 
to 500 volts between the outside wires. There is a decided 
tendency in this direction, and occasional stations have under- 
taken to change to this double voltage, at least to the extent of 
trying 220 volt lamps extensively. At present these lamps are 
of somewhat uncertain quality and rather high price, but they 
have been rapidly improved, both here and abroad. 

It is undoubtedly much harder to get an efficient and durable 
filament for 220 than for 110 volts at a given candle-power. 
Such a filament is necessarily very slender and correspond- 
ingly fragile. If two 110 volt filaments mounted in series 



METHODS OF DISTRIBUTION. 609 

would answer, the task would be simple, but such a combina- 
tion gives double the required candle-power, which is gener- 
ally undesirable. The net result of present experience is that 
while 220 volt lamps can be made to give excellent results in 
efficiency and life they are, as a rule, both poorer and costlier 
than the corresponding lamps of half the voltage. From the 
nature of lamp manufacture this condition is likely to remain, 
in perhaps lessened degree, even when the production of these 
high voltage lamps is extensive. The question between the 
two from a commercial standpoint will ultimately be a close 
one, although at present the advantage is altogether on the 
side of the lower voltage in most instances. The high voltage 
lamps are most satisfactory when of 20 to 32 candle-power and 
worked at 3.5 to 4 watts per candle. Under such conditions 
the filaments, being somewhat thicker than in a 16. c. p. lamp 
of similar voltage, and being worked at a lower temperature 
than the high efficiency lamps, give a reasonably good life. 

Until much experience has been accumulated with reference 
to the high voltage lamps, their use in any considerable under- 
taking cannot safely be recommended. It would be particu- 
larly unwise to attempt it in a large transmission plant, where 
any trouble with the lamps would inevitably be charged against 
the general system. It is better, then, to select for incandes- 
cent lighting a voltage only so high as has been thoroughly 
tried — say 115 to 125. 

The resulting service voltage on the secondary network 
depends on the system of distribution employed. There are 
actually employed for primary or secondary distribution with 
alternating currents about a round dozen of distinct methods, 
more or less convenient and inconvenient, and requiring very 
various amounts of copper for distributing the same amount 
of energy at the same loss and distance. Several of them are 
very convenient and valuable, others have as their only excuse 
for existence the desire to exploit a novelty or to evade some- 
body's patent. 

The simplest of them all is the ordinary two-wire system 
worked with alternating currents. In this the maximum vol- 
tage of the lamps is the maximum voltage of the secondary 
system. To avoid this limitation and to secure the ability to 



610 ELECTRIC TRANSMISSION OF POWER. 

run motors is the principal function of the various modifica- 
tions, polyphase and other, which make up the remainder. As 
these various systems are often exploited, it is worth the while 
to review them briefly, with special reference to economy of 
copper and convenience of installation on a large scale for the 
purpose we are considering. The two-wire system is shown 
diagrammatically in Fig. 298. Its main advantage is extreme 
simplicity. It requires the same amount of copper as a two- 
wire direct current system at the same effective voltage, and 



£ 



Fig. 298. 



is installed in the same general way, except that, owing to the 
peculiarities of alternating currents already explained, very 
large single wires are undesirable and armored conduits must 
be used with great caution, if at all. 

As to motors for such a system, the case is not altogether 
what one would desire. Alternating monophase motors are 
not yet so satisfactory for general service as those of some 
other types, more particularly as regards starting and severe 
service, and, mitil considerable improvement is made in them, 
the pure monophase system is severely handicapped. The two- 
wire arrangement is always at rather a disadvantage in the 
amount of copper required both for feeders and service mains. 

The most obvious modification of this distribution is its evo- 
lution into a three-wire system such as is familiar in Edison 
stations. The extreme working voltage is at once doubled^ 



Fig. 299. 

and thus with the same voltage at the lamps, the cost of copper 
is greatly reduced. If the copper for a given two-wire system 
be taken as 100, that for the corresponding three-wire system is 
31.25, assuming that the so-called neutral wire is of one-half 
the cross section of either of the others. Fig. 299 shows this 
familiar arrangement in diagram. Like every other system 



METHODS OF DISTRIBUTION. 611 

which saves copper, a three-wire distribution is subject to cer- 
tain inconveniences. In the first place, it is necessary to carry 
three wires instead of two over substantially the whole working 
area. Secondly, the lamps must be nearly equally divided 
between the two sides of the system. This balancing of the 
load is not particularly troublesome in a well-managed plant, 
and general experience has shown that the gain in copper far 
outweighs this disadvantage. 

This three-wire distribution has been largely used for alter- 
nating current work. It is sometimes very convenient when 
applied to single or grouped transformers for the lighting of 
large buildings and regions in which balance of load is easily 
preserved. In such case the transformers are supplied from 
high voltage feeders, generally arranged on the two-wire sys- 
tem. As a rule, however, proper balancing is not easy in iso- 
lated districts, and the best use of the three-wire system is for 



Fia. 300. 

a general network of secondary mains, the voltage upon which 
can be controlled from a central station. In an ordinary 
direct current plant, the feeders are of course at low voltage, 
and a great advantage is gained for the alternating arrange- 
ment by feeders at two or more thousand volts supplying the 
mains through transformers. As regards motors, the alter- 
nating current three-wire system is on substantially the same 
basis as the alternating two-wire system. 

More complicated pure monophase systems are seldom used, 
although there is an instance at Portland, Ore., of a four-wire 
feeder system; derived, however, from polyphase generators. 
Fig. 300 shows the arrangement of the lines, which are operated 
in general like a three-wire plant and require similar care in 
balancing, with the additional complication of running four 
wires and balancing three branches. The saving in copper is 
of course very great, the amount needed; allowing half the area 



612 ELECTRIC TRANSMISSION OF POWER. 

of the outside wires for each neutral, being about 16.6 against 
100 for the two-wire plant. The corresponding five-wire sys- 
tem may be passed over, as it is not used at all for alternating 
currents, nor extensively in any way. 

Next in proper order comes the so-called monocyclic system, 
which is essentially a monophase system, but heterophase with 
reference to the operation of motors. Its principal features 
have already been explained. So far as lights are concerned, 
it is simply the monophase system already described in both 
the two-wire and three-wire forms. The '^ power wire," which 
supplies magnetizing current for the fields of the motors, is 
only used in so far as is necessary for its special purpose, and 



Lj 



•r :< 



Fig. 301. 

may or may not form part of the regular network. The two- 
wire monocyclic system shown in Fig. 301 describes itself. 
The expense and trouble of installing the ''power wire" is the 
price paid for the ability to run motors. The total amount of 
copper is, of course, governed by the size and extent of the 
power wire. The main wires must accommodate the full 
current of the generator, for motors and lights must often be 
operated together, and at all events the machine must be fully 
utilized. The power wire, on the other hand, has to carry only 
a part of the current used in the motors. In a system heavily 
loaded with motors, the power wire might be one-half the cross 
section of each of the main wires. If then it extended over 
the entire system, it would add 25 per cent to the copper re- 
quired for the main circuit. Generally its size or extent would 
be less than that just noted. The total copper required for a 
monocyclic system is then variable. Its relative amount may 
vary from 100, when the system is operating lights alone, to 125 
for rather extreme cases of motor load. 

The same general properties hold good for the three- wire 
monocyclic system shown in Fig. 302. It is treated like any 
other three-wire system, except for the addition of the power 



METHODS OF DISTRIBUTION. 613 

wire wherever required. There is evidently a great saving of 
copper over the two-wire monocychc, secured at the cost of 
running an extra wire as a neutral and balancing the load on 
the two branches. The relative weight of copper varies from 
31.25 for lights only to say 40 v/hen the motor system is exten- 
sive. Either form of the system is singularly easy to install 
and operate in plants already having a considerable network 
of lines, since there need be no rearrangement or balancing of 
circuits, but only an additional line wire running to the motors 
installed and extended hand in hand with the motor service. 
The monocyclic system is now very little used in practice, 
however, since it possesses no important advantages over 



vwv^ 



Fig. 302. 

ordinary polyphase systems and is decidedly less satisfactory 
for motor service. 

Passing now to the polyphase systems, it is well to reiterate 
what has already been stated in explaining them, viz., that 
they all involve about the same principles and lead dynamically 
to about the same results. They do, however, differ consider- 
ably in their characteristics as applied to a general system of 
distribution, and in rather interesting ways. 

The diphase system can be worked either with four wires, 
i.e., a complete and independent circuit for each phase, or with 
three wires. The former arrangement is the one almost invari- 
ably used. The two circuits can be worked independently for 
lights, but must be united to allow the operation of diphase 
motors. For the former purpose the two windings of the gen- 
erator may be treated, save in one important respect, like sepa- 
rate monophase alteruators. For the latter purpose they must 
work conjointly. Fig. 303 shows the relations of the two cir- 
cuits. In a general system it is the best plan to carry the two 
circuits throughout the territory to be covered. In this way 
motors can be rim anywhere. Otherwise, if the main circuits 



614 



ELECTRIC TRANSMISSION OF POWER. 



covered different districts, connecting lines might have to be 
run at considerable expense for copper and labor, uniting the 
two systems. Further, when the two circuits are together, it 
is easier to divide the load evenly between them; which is desir- 
able to prevent one circuit of the generator being overloaded 
before the other is fully used. Incidentally, hand regulation 
must sometimes be used for one or both circuits, unless the 
loads are equal as regards drop in the lines. If the generator 
is to be compound wound, the two phases must be equally 
loaded in order that the compounding may be able to hold the 
voltage on both phases alike. It must not be understood that 
unequal loads affect the voltage as in a three-\^ire system — 

1 




Fig. 303. 

they merely produce different ''drops" in the two systems, 
which cannot be equalized by the generator. 

As to the relative amount of copper required, it is, when 
both phases are run together, 100. If separated, this may be 
slightly increased by cross connections for motors. 

A diphase system can be organized with each phase form- 
ing a three-wire system like Fig. 299. This doubles the work- 
ing voltage and so saves copper, but at the cost of very serious 
complication. The full distribution requires six wires, three 
per phase, and these must be carried together or cross-con- 
nected for motors, if separated. The first procedure — run- 
ning two three-wire systems side by side over the same dis- 
trict — involves frightfully complicated wiring; and the second 
if the motors are at all numerous, requires a troublesome 
system of subsidiary lines. In either case, not only would 
each three-wire system have to be balanced in itself, but the 
two must be mutually balanced unless hand regulation is 
resorted to for one or both. 

Altogether the diphase system with separated phases, 
does not lend itself readily to distribution for lights and motors 



METHODS OF DISTRIBUTION. 615 

on a large scale, save in changing over existing monophase 
or other two-wire systems, for which it happens to be exceed- 
ingly well suited. Its worst features are the large amount of 
copper required for secondary mains, and the forbidding com- 
plication of any attempt to secure economy by using the three- 
wire distribution. Like the diphase interconnected system 
about to be described, and certain forms of the three-phase 
system, it is most practicable in plants of moderate size riot 
requiring a complete sub-station with a full system of secon- 
dary mains. 

The interconnected diphase system, Fig. 304, employs a 
common return for the two phases. It has been often proposed 
but seldom used, for a good practical reason. The com- 
bined phases are unsymmetrical with respect to the inductance 




{—i 



Fig. 304. 

of the system, so that, even when the two sides of the system 
are equally loaded, the voltages between the common wire and 
the mains are unequal by an amount proportional to the induc- 
tive loss in the lines. Hence, it is unsuited for long lines 
either primary or secondary, overhead or underground. The 
lamps on the two sides of the circuit are at nearly the same 
voltage, but the voltage between the mains is so compounded 
of the two phases as to give increased working pressure enough 
to reduce the relative amount of copper to 72.8 under the most 
favorable circumstances. The system need scarcely be con- 
sidered further, since it is more curious than valuable, and 
is imlikely to be employed in large sub-station work. 

Three-phase circuits are variously arranged, as has been 
already indicated. The phases are very seldom separated, for 
a six-wire circuit is too complicated for general use, but are 
usually interconnected. The commonest and simplest con- 
nection is shown in Fig. 305. This consists of only three wires 
each running from the terminal of a phase winding on the 



616 ELECTRIC TRANSMISSION OF POWER. 

armature. Motors are connected to all three wires, and 
lamps between any two wires. The voltage is the same be- 
tween each pair of wires, provided each pair be equally loaded. 
The relative amount of copper required is 75, as explained 
elsewhere. Here, as always, the uniting of circuits to save 
copper is accompanied by the need for balancing the loads. 
Not only does change of load on one branch change the drop 
in the other two, but interacts with them in the transformers 
and generators. The disturbance, however, is fortunately trivial 
in amount, except for very great inequalities of load or for 
abnormally large line loss. With ordinary losses in the line 
it is absolutely negligible when the circuits at full load are 
balanced within 10 or 15 per cent, and at light loads far greater 
inequality will have no perceptible effect. With ordinary care 




tJtJLJ 

Fig. 305. 

in arranging the installation the question of balance never as- 
sumes any considerable importance, and need not do so even 
when very close regulation is desired, although extra care is 
necessary in reaching first-class results. The main objection 
to the system of Fig. 305 is the considerable amount of copper 
required for a distribution by secondary mains as compared 
with the ordinary three-wire systems. Its salient advantage 
is its ability to handle motors and lights with equal facility on 
a system composed of only three wires, and with some saving 
of copper. The trouble of approximately balancing the three 
branches is regarded as insignificant by those who are operat- 
ing such systems. This three-phase distribution is often taken 
from the three common junctions of a mesh connection, while 
for motors the connection is a matter of indifference. 

A far better system for sub-station distribution is that 
shown in Fig. 306. It is a three-phase system with a neutral 
wire connected to the neutral point of the three-phase wind- 
ings. The lamps are connected between this neutral wire and 



METHODS OF DISTRIBUTION. 617 

the several main lines. The result is that the working voltage 
of the lamps is the voltage from either line to the neutral point, 
while the working voltage of the system is 1.73 times greater, 
being the voltage between line and line. Hence, there is a great 
reduction in the amount of copper required, the relative weight, 
as compared with the two-wire monophase system, being only 
29.2 if the neutral wire is taken of cross section equal to one- 
half that of either of the other wires. This system must be 
balanced approximately, but requires less care in this respect 
than the ordinary three-phase connection just described. It is 
on the whole, better adapted for large distributions of mixed 
lighting and power than any other of the modern alternating 
systems, since it combines a fairly simple arrangement of wiring 
with very great economy of copper. It lends itself readily even 
to underground service, giving a rather simple cable construc- 




FiG. 306. 

tion and facilitating testing. It is used with excellent results 
in the Folsom-Sacramento, the Fresno, and other important 
transmission plants, for the main work of distribution. 

An interesting modification of the three-phase system is that 
used in the city of Dresden and shown in Fig. 307. Here the 
system is constituted in the ordinary way, but two of the leads, 
a and h, are arranged to carry all the lighting, while the third 
wire c, which may be of much less area, is used only in connec- 
tion with the motors. It may even sometimes be advantage- 
ous to increase the cross section of two of the armature wind- 
ings at the expense of the third. A machine so constituted 
would have fully as great capacity as a monophase machine 
of the same dimensions, and still would be amply able to carry 
any ordinary motor loads. Even with the ordinary three- 
phase winding this connection may be used without serious 
reduction of output as compared with a monophase generator 
of the same cost. Obviously the relative copper required may 



618 



ELECTRIC TRANSMISSION OF POWER. 



vary from 100, when the load is of Ughts only, to 75 for the 
other extreme case. With half lights and half motors it would 
require 80-90 relative copper, according to the allowances 
made for drop, inductance, etc. In point of convenience it is 




et^ 



Fig. 307. 

very similar to the "monocyclic" system, and like the latter 
may be used with great ease in remodeling monophase systems 
for motor work, without requiring special generators of a type 
which is tending to obsolescence. 

A natural derivative of this mixed system is shown in Fig. 
308. It is a combination of Figs. 306 and 307; a and h being 
the mains, c the motor wire and d the neutral wire. The rela- 
tive copper required naturally varies with the proportion of 
motors and lights; 36 representing that necessary for an ap- 




• 5 



Fig. 308. 



proximately equal division under ordinary conditions. Fig. 
308 may be compared with Fig. 302, the monocyclic three-wire 
system. It is about the same in effect as the three-phase 
system with neutral, having but two branches instead of three 
to balance, and paying for this privilege with about 20 per cent 
more copper. 

There is thus a liberal choice of methods more or less avail- 



METHODS OF DISTRIBUTION. 619 

able for the general distribution of power and light. Any one 
of them may prove to be the most useful in particular situa- 
tions. Now and then it may be worth while to use more than 
one of them in the same plant, as, for example, monophase two- 
wire and monophase three-wire or three-phase and three-phase 
with neutral. 

It must be borne distinctly in mind that one cannot organize 
a large sub-station distribution successfully on any substan- 
tially two-wire system — the cost of copper is too great. If 
work akin to that of a large central station is to be done, 
methods must be used akin to those which have proved suc- 
cessful in such work. The methods of distribution must be 
those which are capable of giving a secondary network of mod- 
erate cost, easy to install and maintain. The use of alternating 
current gives a great advantage in the use of high tension 
feeders and in efficient methods of regulation, and there is at 
present no difficulty in furnishing a reliable and efficient motor 
service; but to secure the full advantage of all this, one must 
cut loose from the traditioas of alternating current service. 
A transformer must be looked upon not merely as a device for 
lowering the voltage to a point available for direct consump- 
tion, but as a generator of extreme simplicity and enormous 
efficiency that operates without attention, can be started and 
stopped from any convenient point, and may be regulated 
without material loss of energy. That it receives current from 
a transmission line instead of energy of rotation from a steam 
engine is clear gain in simplicity, not a marvel to be looked at 
askance. On the contrary, the transmission plant is usually 
quite as manageable and trustworthy as a steam plant. 

Approaching the sub-station from this standpoint, the prob- 
lem of effective distribution becomes tolerably straightforward. 
Given the transmitted energy, it must be distributed over a 
knowTi area cheaply and efficiently, with the smallest feasible 
loss of energy at all loads, and the best regulation attainable. 
It will not do to plead transformer losses when the lights burn 
dim, or the depravity of alternating motors when they flicker. 

First, as to locating a sub-station. On general principles 
any station should be placed as nearly as may be at the centre 
of its load, and inasmuch as a transformer station requires 



620 



ELECTRIC TRANSMISSION OF POWER. 



little space and makes no noise, there are few limitations to its 
position save the ability to bring to it the transmission lines, 
which, being generally at very high voltage, will be eyed cau- 
tiously by the mimicipal authorities. The main district to be 



Fig 309. 



covered is generally quite definite, and the next thing to be 
done is to reach every part of it with a network of working 
conductors proportioned to the service. The nature of the 



3\ 



^.V^' 



^\-''- 



<5t- 



i* 



Fig. 310. 



wiring will vary, according to the system employed; but the 
generally accepted principles are, save for inductance, the in- 
fluence of which has already been considered, the same that 
are familiar in continuous current work. 



METHODS OF DISTRIBUTION, 621 

The problem is to supply a certain amount of energy at a 
given loss over a laiown area, and the formulae already stated 
give the key to the solution. Working out the details, how- 
ever, is a somewhat complicated matter, requiring great judg- 
ment and finesse, and to be accomplished properly only by an 
experienced engineer, working on the spot. The intricacies 
of the problem are too great to be treated in an elementary 
treatise like the present. The general situation, however, is 
something as follows: A city, Fig. 309, is to be supplied with 
light and power from a transmission plant. Let A be the 
centre of load at which the transmission lines terminate. At 
this point can most advantageously be located the reducing 
sub-station, lowering the voltage of transmission to perhaps 
2,200 volts for feeders, or to a tenth of this for direct supply. 
The centre of load considered is not the geographical centre 
of the district to be supplied, but the centre of gravity of the 
load. This is determined just as if the electrical loads at 
various points were weights fastened on a rigid weightless 
framework. For example, suppose there are given the loads 
of Fig. 310, five in number and in relative magnitude as shown 
by the figures. Connect any two of them, as 1 and 2. These 
would balance as weights at the point a, which acts with re- 
spect to other points as if 1 and 2 were concentrated at it. 
Now connect a and 3. These weights are equal, hence the 
point of balance is the middle point of a 3, b, at which the 
weight is evidently 6; 6 4 balances at c, where the weight is 10, 
and finally the whole system balances at d, which is the centre 
of gravity. The points may be taken in any order, but each 
line must be divided so that, for instance, the length a 1, mul- 
tiplied by weight 1, shall equal the length a 2 multiplied by 
weight 2. 

The centre of load thus found should be the centre of dis- 
tribution to secure maximum economy in copper. The fact 
that distribution lines usually ruQ in a rectangular street sys- 
tem renders the solution thus obtained merely approximate, 
but it is nevertheless close enough for purposes of station loca- 
tion. 

Recurring to Fig. 309, several methods of arranging the 
service are available. The simplest is, if the load is tolerably 



622 ELECTRIC TRANSMISSION OF POWER. 

concentrated, to institute a secondary network about A so as 
to include a good part of the load and then pick up the out- 
lying load by transformers, placed where they can do the most 
good, fed from high tension feeders. Sometimes, however, 
there will be no heavy service near the centre of load, so that 
the whole work of the station will be done through high ten- 
sion feeders, each supplying through its transformers a more 
or less extensive system of secondaries. Standard trans- 
formers are commonly wound for about 2,200 volts primary, 
but 2,400 volts and 3,100 volts are also regular primary pres- 
sures and the former is in considerable use. Above these 
figures small transformers are rather expensive, but if necessary 
the standard transformers can be used in star connection. 

As has already been pointed out, there is every reason for 
using a secondary network, connected directly to the reducing 
transformers, at the sub-station if possible, thereby avoiding 
the expense of transformers for a second reduction in voltage 
and the loss of efficiency involved in such a reduction. The 
house-to-house transformer distribution should be shunned 
as one would shun the plague, if there is any expectation of 
securing an efficient station, capable of giving first-class 
service. 

It must be remembered that to be successful, a modern plant 
for distributing power and light throughout a city must be 
able to compete with the best that can be done, not with the 
precarious and shiftless service of a dozen years ago. 

It is possible with a modern alternating plant, to equal the 
best service given by a continuous current central station, but 
the feat can be accomplished only by the study of central 
station practice. 

The sub-station at A, Fig. 309, should be treated, so far as 
distribution is concerned, as if the reducing transformers were 
ordinary generators. The transformer units should be of the 
size that would be convenient if they were generators, and the 
bank should be so managed as to keep the transformers in use 
as thoroughly loaded as possible. From the transformer bank 
should run feeders to the principal sub-centres of distribution 
in the network, with pressure regulators in such of the feeders 
as require them. From these sub-centres, pressure wires should 



METHODS OF DISTRIBUTION. 623 

run back to the station whenever needed for the guidance of 
the operator in charge of the regulators. 

Outside the effective radius of distribution of the principal 
secondary network will come the independent sub-centres 
referred to, with their high tension feeders and subsidiary net- 
works. These latter should be, so far as possible, interlinked 
so that, at times of light load, only the transformers actually 
needed shall be in service. If secondary pressure wires are 
brought home from the subsidiary networks, all the regulation 
can be done on the high tension feeders, thereby giving equally 
good service all over the plant. Most continuous current sta- 
tions extend their lines far beyond the radius that is economi- 
cal for low tension currents, and often have to depend on 
boosters with feeders worked at a heavy loss for service in 
the outlying districts. With an alternating system this diffi- 
culty is avoided, and the loss in transformers and regulators 
is far less than that incurred with boosters and long low ten- 
sion feeders. 

As for the motor service in such a system, it should be treated 
by common sense, as it would be in a central station distribut- 
ing continuous current. 

Alternating motors, polyphase or other, can be connected 
to the secondary mains up to the point at which their demands 
for current become burdensome. At that point the mains 
must be reinforced or special feeders run, just as would be the 
case with continuous current motors. The only difference is 
that produced by the so-called idle current in the alternating 
motors, which simply means that the point in question is 
reached a little sooner than with continuous current motors. 
In practice this difference need not be enough to be of serious 
moment in plants having the ordinary proportions of lights 
and motors. In case of large motor plants in which the ser- 
vice is severe, the use of special high tension feeders will relieve 
the trouble that might be experienced with the lights, but this 
expedient is one to which recourse would seldom have to be 
taken on a large scale. 

The greatest difficulty in such sub-station distribution is, as 
has been already indicated, the arc lighting. At present the 
alternating arc lamp is hardly adequate to meet all conditions^ 



624 ELECTRIC TRANSMISSION OF POWEB. 

although it is coming gradually into more and more extended 
and successful use. 

In cases where power is to be supplied for railway purposes, 
there are few difficulties in the way. Existing railway gen- 
erators can readily be utilized by driving them from syn- 
chronous motors. This is the method employed in the old 
transmission to Sacramento, Cal., and elsewhere not infre- 
quently. Where the utilization of the old machine is not 
important, or in new plants, the tendency is to use the rotary 
converter, which has been already fully discussed. Such 
apparatus was first put into extensive use in the Portland 
(Ore.) transmission plant, and is now largely and very suc- 
cessfully employed everywhere. Continuous current for other 
purposes may be obtained with ease by the various methods 
described in Chapter VII. A very instructive example of 
recent practice in sub-station distribution may be found in 
Salt Lake City, Utah. This city is supplied with electric 
power from a group of transmission plants, the general loca- 
tion of which is shown in Fig. 311. These plants were started 
independently, but later were consolidated with the local light- 
ing interests and are operated together. The Big Cottonwood 
plant, started in 1896, contains four 450 KW, three-phase 
generators, and has a double 10,000 volt circuit 14 miles 
long into Salt Lake City. The Ogden plant, started the suc- 
ceeding year, has five 750 KW three-phase generators, at 
2,300 volts, at which pressure energy is supplied in the city 
of Ogden. The rest of the output is raised to 16,000 volts 
and sent into Salt Lake City over a pair of circuits 36 J miles 
long. 

The third plant, that of the Utah Power Co., is like the 
first, in the Big Cottonwood Caiion, but is two miles nearer 
the city, and contains two 750 KW two-phase generators, with 
a two-phase-three-phase raising bank of transformers to 16,000 
volts, feeding duplicate three-phase circuits. 

These are now (1905) also interlinked with the Provo system 
with its plants on the Provo River and with a 2,000 KW plant 
at Logan, some 40 miles to the north of Ogden. The longer 
lines are worked at 40,000 volts. The whole system comprises 
six hydraulic plants, two auxiliary steam plants, and 420 miles 



METHODS OF DISTRIBUTION. 



625 




Fig. 311. 



626 



ELECTRIC TRANSMISSION OF POWER. 



of high voltage Hnes covering a territory 160 miles long. The 
entire network is successfully operated in parallel. 

The Utah plant, with one generator and line of the Big 
Cottonwood plant are put in parallel on the high tension side, 
and run two-phase rotaries in a sub-station near the centre of 
the city. This sub-station supplies power to the electric rail- 



ID DDQD DDE 

]DDDnnn ' 



2000 Volt Feeders 3 wire 

2000 "Volt Mains 3 wire ^^ 



uUuuuuuUUuLi, 

DDpnanDDDDD 

DDDDDDDnDnn 
DDnnnDDDDDD 

udddddddddddcs 




Fig. 312. 



way system, and is entirely separate from the lighting distri- 
bution. 

The Ogden lines and the remaining Big Cottonwood line are 
put in parallel on the low tension 2,300-volt side, at a centrally 
located sub-station devoted to lighting and power. From this 
sub-station is carried out a system of three-phase primary 
feeders and mains serving the entire city. This network is 



METHODS OF DISTRIBUTION. 



627 



well shown in Fig. 312. The primaries are connected in mesh, 
but the secondaries have the star connection with neutral, 
forming a regular three-phase four-wire distribution, with 115 
volts between the neutral and either phase-wire. Motors are 
connected to the three-phase wires, giving about 200 volts, and 
all motors over 10 HP are put on transformers of their own. 



nnaQDnnn 



Secondary Mains - 4 wive — .^-^^— 
Secondary Mains - 2 wire =^^= 
Transformers represented by dotts=^"= 
Transformer not shown on 4 wire mains. 



1 



npn™.; 
nnaorli 
nTi 



.ILILIUUUUUUULIU 
^aDDDDDDDDDn 

, jODDnnnnnDDa 
irDDaDDDDdnDDn 
DDDDnnDDannaa 



3DgnnriijnL' 
, lonHyi iiii lUi/ rii 

UJUUuuuQm 



-'Tij-tlncihrTi 







r1, 



R^riDDDDDD I g l"l 

"Q; an □ fj □ □ □ n €i"irn"Ti 
gnngnnaDui'LL-Li.i 

(Izi f~i n n n n f— r n n r-,rnllpLi 

iBiijipn &n 



nnanc 

.,.t- □ D □ □ □ □ □ DiGiiDC nnnnoD D on n 

' Hdddd 
ILIdqdd 




Si 



D 



n 



Jioriun 



n 



□□ 



Fig. 313. 

As appears from the cut, Fig. 312, the primary network is 
quite symmetrically arranged with reference to the extension 
of service. 

The secondary service is developed into a systematic net- 
work of mains, well shown in Fig. 313. Where the service is 
dense there is a regular four-wire network. Each block is served 
by two groups of three transformers at the opposite comers, 



628 ELECTRIC TRANSMISSION OF POWER. 

from which secondary mains are carried around the block and 
tied by fuse boxes to the secondary mains of adjacent blocks. 
When a distribution of this sort is used, fuse boxes and cut-outs 
should be judiciously employed on both primary and secondary 
networks, so that in case of severe short circuits or of fire the 
district affected can be promptly cut clear of the rest of the 
system. 

Where service is less dense, as in residence districts, the first 
step is to put in a two-wire secondary main from the trans- 
formers, consisting of a neutral and one phase-wire, street being 
balanced against street in this light service. Then when busi- 
ness demands, the other phase-wires are carried into the street, 
lights balanced upon them, and the completed four-wire system 
is then tied to the network already completed. 

Commercial arc lighting is by constant potential alternating 
arcs, of which some 500 are in use in Ogden and Salt Lake 
City. Street lighting is at present supplied from continuous 
current series arc machines driven by synchronous motors. 
These motors are located in an old electric light station near 
the sub-station, and can be driven as generators in case of 
need, while the sub-station itself has a small reserve steam 
plant and generator equipment. The synchronous motors are 
useful in regulating the voltage at Salt Lake City, being capable 
of accomplishing a variation of 10 per cent when the lines are 
heavily loaded. 

This scheme of sub-station distribution is admirably con- 
ceived, and works out very simply and neatly. The trans- 
mission system itself is decidedly complex, owing to the vari- 
ous and diverse power houses, but it works well and has done 
excellent service. It is interesting to note that no trouble 
is experienced in running these distant and diverse plants in 
parallel. At light load there is some interchange of current, 
but at heavy loads everything settles down to business. 

All the stations are connected by telephone, and by a little 
intercommunication the generators can be put in parallel in 
the ordinary manner either at a station or at the sub-station in 
Salt Lake City. The record of the system for continuity of ser- 
vice has been good, and it is worth noting that mo-it instances 
of trouble on the lines have been due to malicious interference, 



METHODS OF DISTRIBUTION. 629 

such as shooting off insulators and throwing things across the 
hnes. Altogether the system is a notable instance of the flexi- 
bility and convenience of modern power transmission methods, 
as well as a good example of a systematic and logical develop- 
ment of the distribution system. As the service grows various 
refinements will doubtless suggest themselves, but the system 
is correctly started and there will be little work to undo. It is 
in striking contrast with some transmission systems which 
could be named, in which the operators, less skilled in dealing 
with modern methods, have blundered around trying to give 
good service in an unsystematic and helter-skelter fashion, 
getting deeper into trouble at every jump, and then blaming 
the state of the art for the results of their own lack of discretion. 

The most delicate and important work in connection with 
heavy sub-station service is that involved in the proper regu- 
lation of the voltage. The sub-station receives its supply of 
energy often from a long transmission line ia which there is 
considerable drop, to say nothing of that encountered in the 
generators and two banks of transformers. 

It must distribute this energy throughout a complicated 
network, so that the variations in pressure at the lamps shall 
not exceed two or three volts at the outside. This is never an 
easy task — it tries the ingenuity even of the best central 
station engineers. 

In connection with a transmission plant, probably the best 
plan is to divide the regulation into two stages : first, that con- 
cerned with the transmission proper,tand second, that concerned 
with the distribution. By compounding the generators, or by 
hand or automatic regulation of generators having good inher- 
ent regulation, it is certainly possible to hold the voltage closely 
constant up to the primary terminals of the reducing trans- 
formers. In large alternating generators ordinary compound- 
ing is seldom or never attempted, and in many cases the sole 
reliance is hand regulation, which is by no means to be despised 
in the absence of other means. 

Within the last few years several automatic regulators 
capable of giving excellent service have been brought out, and 
they are coming into somewhat extensive use. The two prin- 
cipal forms have already been described. 



630 



ELECTRIC TRANSMISSION OF POWER. 



None of these regulators are arranged automatically to take 
care of the line drop when the power factor varies considerably, 
but they are amply sufficient to provide for the general regula- 
tion up to the sub-station, at which point it may be taken up 
as a separate problem. This residual regulation ordinarily 
consists of the drop in the reducing transformers, which should 
be not over 2 per cent; the drop in the feeders and secondary 
mains; in high tension feeders and transformers when em- 
ployed ; and finally in the house wiring. These losses will aggre- 




FlG. 314. 



gate generally less than 10 per cent, and are best cared 
for in the sub-station. As the variations in load, and hence 
in loss, are generally rather slow, this regulation should be 
accomplished without difficulty. In some cases it may be 
advantageously reduced in amount by carrying the primary 
regulation through to the secondary terminals of the reducing 
transformers. 

However this may be, the regulation of the voltage on the 
secondary lines must be carried out with the utmost care. 



METHODS OF DISTRIBUTION. 631 

The apparatus employed for this purpose is both very simple 
and exceedingly efficient. It is in every case a transformer 
arranged to give a variable ratio of transformation and adding 
its E. M. F. to that of the working circuit. 

The best known form of this device is probably the Stillwell 
regulator, which has for some years past been very successfully 
used by the Westinghouse Company. It is, in effect, a trans- 
former, from the secondary coil of which leads are brought out 
to terminals so arranged as to enable one to vary the number 
of secondary turns, and so to vary the E. M. F. added to the 
working circuit. Fig. 314 shows a diagram of the connections 
by which this result is effected. The diagram is self-explana- 
tory, except that it should be noted that the ''preventive coil" 
is intended to avert the necessity of breaking circuit or short 
circuiting a secondary coil in passing from one contact to the 




Fig. 315. 

next, and that the reversing switch enables the regulator to 
diminish the voltage on the working circuit, which may now 
and then be convenient. In the ordinary practice of the West- 
inghouse Company, this regulator is installed in the generating 
station and used to vary the voltage on the primary line. In 
sub-station work it can be applied either to the primary or sec- 
ondary side of the reducing transformers; practically the latter 
is the working connection. These regulators are made to have 
a range of action of 10, 15, and 20 per cent of the working vol- 
tage. They are generally employed with a very ingenious 
device known as the ''compensator," the function of which is 
to indicate the pressure at the end of the line or feeder without 
the use of pressure wires. The principle of this is shown in Fig. 
315. The voltmeter V is in circuit with the opposed E. M. F.'s 
of two secondaries C and D, of which the primaries A and D 
are respectively in series and in shunt with the load. The 
voltage of D is proportional to the main primary E. M. F., that 



632 ELECTRIC TRANSMISSION OF POWER. 

of C to the primary current strength, so that the difference 
between C and D, which shows on the voltmeter, can be made 
proportional to the voltage as reduced by the drop due to the 
current in the line. The compensator is in addition provided 
with a series of contacts by which the E. M. F. of C is adjustable 
for any given percentage of loss in the line. 

The practice of the General Electric Company is somewhat 
different. The generator is generally over-compounded for a 
fixed loss in the line at full load, or hand regulation is effected 
by the field rheostat. For sub-station purposes a variable 
transformer is employed to vary the working voltage. The 
principle of this voltage regulator is the variation of the induc- 
tive relation of primary and secondary instead of varying the 
number of secondary turns. The apparatus itself is made in 
several forms, one of which, used in a number of three- 
phase plants, is shown in Fig. 262. It is essentially a trans- 
former with a movable secondary, and serves either to raise or 
lower the working voltage, as occasion requires. The grada- 
tion of voltage is not by definite steps, but by continuous varia- 
tion. The apparatus is made for substantially the same range 
of action as the Still well regulator just described, and accom- 
plishes the same result. The General Electric Company also 
makes a voltage regulator with a variable number of secondary 
windings. 

It should be stated that neither over-compounding nor any 
similar devices can deal successfully with a load of very variable 
power factor such as is often found in motor service. They 
can be made to work well on either non-inductive or inductive 
load, but are not well adapted for a load of which the power 
factor varies much. For this condition nothing has yet been 
devised so good as pressure wires combined with intelligent 
hand regulation. 

Various attempts have been made to employ pressure wires 
in conjunction with automatic regulators, but none have yet 
met with very encouraging success. Automatic control of 
alternating current sub-station regulators is by no means so 
simple a matter as pressure regulation applied to the generator. 
Apparatus of the type of the Stillwell regulator has to deal 
with fairly large currents, and the contact arm^ to prevent undue 



Methods of distribution. 



633 



load on the '^preventive coil/' should be quickly moved from 
segment to segment. This involves various mechanical diffi- 
culties and the expenditure of some power. Apparatus like 
Fig. 316 works none too easily, on account of the magnetic 
forces involved. Such regulators are sometimes motor-driven 
and thus readily controllable from the switchboard. In fact, 
it is safe to say that the problem of working sub-station regu- 
lators automatically involves the use of powerful relay mechan- 




FlG. 316. 



ism akin to that used for water-wheel governors, although on 
a very much smaller scale. 

No such apparatus is just at present in practical use, 
although if successful it would be in considerable demand. Still 
less progress has been made toward the development of an 
automatic balancing device for polyphase circuits. Given a 
good automatic sub-station regulator, and its application to 
preserving accurate balance in a two-phase or three-phase 
distributing system is an obvious extension of its general use. 
Balance is not difficult to secure with a little tact in arranging 



684 ELECTRIC TRANSMISSION OF POWER. 

the load, but sometimes when there is a particularly heavy 
lap load, there will be sensible unbalancing while this load is 
coming on and going off. This is taken care of sometimes by 
having certain loads that can be switched at will upon any leg 
of the circuit, and sometimes pressure regulators are installed 
for manual operation. A good automatic balancer and pres- 
sure regulator would often be of very considerable service, but 
it is not yet forthcoming. It must not be supposed that the 
lack of it is a very grave deficiency, since practically all ordi- 
nary central station regulation is manual save in so far as it 
can be accomplished by over-compounding the generators. 

The devices just described are amply competent to furnish 
very exact regulation for sub-station purposes. Its complete- 
ness depends in the last resort on the skill with which the dis- 
tributing system is designed. If this is carefully done, the 
sub-station regulation should hold the voltage within very 
narrow limits clear up to the lamps. 

As regards the best S3^stem of transmission to employ in con- 
nection with heavy sub-station work, there is naturally a wide 
diversity of opinion. In the author's judgment, there is at 
present no distributing system for large sub-station work in 
connection with long-distance transmission so generally advan- 
tageous as the three-phase distribution with neutral wire shown 
in Fig. 306. It is remarkably free from trouble as regards 
balancing, and extraordinarily economical of copper. With 
further advance in the development of single-phase alternat- 
ing motors, the single-phase three-wire system shown in Fig. 
299 will do admirable work when the motor service is rather 
light. The diphase system has been installed in some central 
stations and the "monocyclic" in others, so data will eventu- 
ally be available regarding each of these systems, but there is 
little reason to expect as good general results as could be ob- 
tained by the systems mentioned above. Diphase, mono- 
cyclic, and the Dresden three-phase systems are, however, very 
much easier to adapt to the circuits of present stations than is 
the three-phase system with neutral wire. 

When a large part of the output of a transmission plant is 
required for railway work and other motor service of extreme 
severity, and a lighting system is also to be operated, it is a 



METHODS OF DISTRIBUTION. 635 

wise precaution to work the two services normally over sepa- 
rate lines and from separate generators as is done in the Salt 
Lake City system just described. Otherwise the variations 
of load may be so great and so rapid that no care in regulation 
could prevent serious fluctuations in voltage. A small rail- 
way load and all ordinary motor service can be worked from 
the same circuits as lamps without much difficulty. These 
limitations are not peculiar to transmission plants — no Edison 
station, for instance, would dare to attempt working a low 
voltage conduit railway from its lighting mains. In these, as 
in many similar matters, a little common sense will prevent 
serious mistakes and show the necessity of working every sys- 
tem so as to obtain the best possible results, and not to discover 
what it will endure without giving intolerably bad service. Of 
late storage batter}'- auxiliaries have often been suggested, and 
sometimes have been employed, in connection with power 
transmission plants. Some reference has already been made 
to storage in Chapter II, but the matters here to be consid- 
ered are of a different character. In transmission work a 
battery may be used for two entirely distinct purposes. In the 
first place it may be used, as it sometimes is in steam-driven 
stations, for the purpose of storing energy at times of light load 
to be used in making up deficiency of power at times of heavy 
load. 

In steam-driven stations the installation of a batter)^ effects 
a considerable economy by enabling *the engines to be run at 
all times at the points of maximum economy, and an additional 
saving, in first cost, by reducing the capacity of the steam plant 
and generators required. The conditions of economy depend 
mainly upon local circumstances, but a material saving can 
be made in many instances by using the battery. 

In hydraulic practice the case is different. In the average 
water power plant the main hydraulic works should generally 
be installed for the full available capacity, save in the few in- 
stances when a partial fall can be economically utilized. As 
a rule the dam will be substantially the same for a partial de- 
velopment as for a complete one, and the latter can be carried 
out more cheaply at the start than when added as patchwork 
later. Consequently there is seldom or never any saving in 



636 ELECTRIC TRANSMISSION OF POWER. 

installing a costly batter}'- subject to heavy depreciation in 
order to avert the first cost of a larger plant. Further, the loss 
of energy in the battery is much greater than the loss ordi- 
narily incurred in the hne at full load, so that the total saleable 
power for a given first cost would in nearly every case be re- 
duced by installing a battery. The one case in which a battery 
can advantageously be used in connection with power trans- 
mission for the purpose indicated, is that in which the total 
hydraulic power available is actually insufficient to carry the 
required maximum load. Storage may then be very advan- 
tageous, since is enables the unutilized power at light load to 
be applied to the peak. Especially will it be advisable when 
the peak is high and the load factor rather poor, under 
which conditions a battery may raise the possible max- 
imum output by 30 to 50 per cent, sometimes even a little 
more. 

The second use of a battery is as a reserve to tide over a brief 
break down. The question of reserve against accident in trans- 
mission work is always a troublesome one. In the author's 
opinion the need of a complete reserve located in the sub- 
station is overestimated. Experience clearly indicates that of 
the interruptions of service occurring on the system of a trans- 
mission plant with sub-station distribution, only a very small 
minority occur on the transmission line proper. The distribu- 
tion lines throughout an average city are peculiarly exposed 
to interruption from limbs of trees, which in residence streets 
can never be adequately trimmed; from the fall of foreign 
wires; from necessary cutting off in case of fire, and from other 
causes. A high voltage transmission is neither more nor less 
likely to encounter trouble on its distributing system than an 
ordinary central station. So far as these causes of trouble go, 
the transmission plant's sub-station is exactly on a par with any 
other central station in requiring special precautions. Now 
while central stations always should have more or less reserve 
apparatus to use in case of break down, it is not required on 
account of possible trouble on the line except as such trouble 
may injure apparatus. A short circuit on the feeding system 
will not be removed by the presence of a spare engine and 
dynamo in the station. Hence, the need of reserve in the sub- 



METHODS OF DISTRIBUTION. 637 

station of a power transmission system bears relation simply to 
the accidents which may affect continuity of service as regards 
the main transmission line, and particularly accidents producing 
more than momentary interruptions. Such accidents are rare 
on properly designed and erected lines, and save on extremely 
long lines of which the cost is a considerable part of the total 
cost of the system, it is generally true that a fraction of the 
cost of a complete reserve plant at the sub-station would pro- 
vide a duplicate line so guarded that reserve apparatus would 
be practically needless. With well-built duplicate pole lines 
and proper switchhig arrangements, serious trouble on the lines, 
save under conditions which would also paralyze the service 
on the distributing system, and thus cripple the plant in any 
event, becomes almost impossible. 

Sometimes, however, a partial auxiliary plant is extremely 
useful, but it is rather for its convenience in case of repairs 
to apparatus at the generating station or sub-station than as a 
safeguard to the main line. In working a large sub-station, 
a storage battery may be of considerable use in this way, par- 
ticularly if the system is being pushed near to its capacity. It 
is decidedly not good policy, however, to use a battery unless 
the station is upon a scale large enough to warrant the employ- 
ment of an especial man skilled in handling batteries and un- 
burdened with other duties. Charged and discharged through 
motor generators or rotaries, a storage battery can be put into 
service on a moment's notice, and is far less troublesome to 
keep up than any other auxiliary for temporary use. 

In some localities a generator coupled to a gas or oil engine 
makes an admirable auxiliary. Such engines can now be ob- 
tained of large output and very high economy, and form a re- 
serve almost as convenient as a battery. Steam reserves are 
not large in first cost, imless high economy in operation is 
attempted, but cannot be put quickly into action unless the 
fires are kept banked, which is a very considerable expense. 
However, by keeping a banked fire under threatening climatic 
conditions the reserve can be ready when it is likely to be 
needed, and if apparatus needs repair there is generally notice 
enough given to get steam up. Power of quick firing is of 
great importance in boilers for an auxiliary plant, and with 



638 ELECTRIC TRANSMISSION OF POWER. 

tactful treatment a steam reserve is probably the most satis- 
factory for plants of moderate size. 

In an increasing number of cases a steam auxiliary plant is 
used to supply a deficit of power at times of low water. The 
more use required of such a plant the tnore regard must be 
had for high economy, in which respect it must be sharply 
distinguished from an auxiUary used merely to tide over emer- 
gencies and accidents. 



CHAPTER XVI. 

THE COMMERCIAL PROBLEM. 

Power transmission is of little avail if it does not pay, and 
the chances of commercial success form the first subject of 
investigation in the development of any power transmission 
enterprise. Reduced to its lowest terms, the question presents 
itself thus: Can I profitably furnish power at a price which will 
enable me to undersell the current cost of power production? 
Evidently this question cannot be answered a priori, but must 
be thoroughly investigated in each particular case. 

The first thing to be determined is the existence of a suffi- 
cient market, the second thing is the price current in this 
market. It is not difficult to find out the gross amount of 
power used in a given region, but it is exceedingly hard to dis- 
cover the real cost of production. Even if all men were strictly 
veracious it is a fact that very few users of power have any 
clear idea of what they pay for it. Coal bills and wages are 
tangible and men realize them, but interest, depreciation, 
repairs, miscellaneous supplies, water, taxes, insurance, and 
incidentals, are seldom rigorously charged up to the power 
account, and these are large items when power is used irregu- 
larly. 

Further, the cost per HP is often computed from the nominal 
HP of the engine, without exact knowledge of the real average 
yearly load. Hence, people often think that they are produc- 
ing power at $15 or $20 per HP per year when the real cost is 
$30 to $50. 

The most exhaustive researches as yet made on this subject 
are those of Dr. C. E. Emery. The accompanying table gives 
a summary of his results, based oq 500 net HP delivered for 
ten hours per day, 308 days in the year. The power is sup- 
posed to be derived from a single engine worked continuously 
at its normal capacity. These figures represent results much 
better than are generally reached in practice, since most en- 

639 



640 



ELECTRIC TRANSMISSION t^F POWER. 



gines are not worked continuously at full load. In a large 
majority of cases the real cost exceeds that given in the table, 
even for engines of similar size. For the rank and file of small 
engines used for miscellaneous manufacturing purposes, cheaply 
built and generally underloaded, the tabular figures should 



Kind of Engine. 


Coal 
|2 per T. 


Coal 
$3 per T. 


'Coal 
$4 per T. 


Coal 
$5 per T. 


Simple high speed .... 
Simple low speed .... 
Simple low speed, condensing 
Compound condensing, low 

speed 

Triple expansion condensing, 

low speed 


$29.81 
28.46 
22.82 

21.97 

22.35 


$36.17 
34.20 
26.77 

25.53 

25.32 


$42.54 
39.94 
30.73 

29.09 

28.28 


$48.90 
45.67 
34.69 

32.65 

31.25 



be nearly doubled. In regions where coal is unusually dear 
the cost in units of 50 HP and upward may range from $100 
to $150 per HP year for a ten-hour day. Costs considerably 
below those in the table are now and then reported, particu- 
larly from engines in textile mills where the load is especially 
favorable. Some of the reduction is undoubtedly due merely 
to bookkeeping, a portion of the expense properly chargeable 
to power being taken care of elsewhere, but some very low 
genuine costs have certainly been secured. Dr. Emery's 
tables are based on costs which can be materially lowered at 
present prices as regards certain items, and they include some 
items of expense which in favorable cases can be reduced. 
For example, in the case of large engines the labor cost is 
materially less than with the 500 HP assumed, and the inter- 
est charge for an engine considered as part of a manufactur- 
ing plant might properly be reduced to 5 per cent. 

Then the table is based on average steam consumption, 
while in recent mill engines a better figure is justified. 

Assuming a power of 1,000 BHP and coal at $2.00 per long 
ton, and making the necessary modifications in the data as 
just indicated, the cost of the HP year on the basis of 308 days 
of 10 hours each per year, with first-class compound condens- 
ing engines, falls to about $17 to $18. These figures have un- 
questionably been reached in actual practice, although rather 



THE COMMERCIAL PROBLEM. 641 

seldom. They must, however, now and then be reckoned 
with, and can be met only by very carefully planned trans- 
mission from an unusually cheap water-power. As a rule, even, 
in large engine plants, the cost per HP year of 3,080 hours runs 
above rather than below $20. On variable load the costs are 
likely to rim 20 or 25 per cent higher. There are few cases in 
which transmission from cheap water-power on a large scale 
cannot beat out steam power even in large units. 

In units under 50 HP one is very unlikely to find the HP 
year, reckoned on the above basis of 10 hours per day, costing 
less than $50, even with coal as low as $2 per long ton. These 
are the facts in the case; the fancies will be duly appreciated 
if one canvasses for electric power. Not more than one man 
in six knows and will admit that his power is costing him as 
much as the table would indicate. The process of reasoning 
(so called) is often about as follows: ''I paid for my engine and 
boiler house when I built the factory, and I do not propose to 
charge my engine rent. It has been running ten years and is 
just as good now as it ever was; has not depreciated for my 
purpose a cent. If any repairs were needed, the engineer and 
one of my men have made them and they haven't cost me any- 
thing but my material. My fireman I have to have anyhow, 
for I heat by steam, and my taxes and insurance I have to pay 
anyhow: that is a 200 HP engine; my coal cost me $2,450 last 
year, and oil and stuff $70. I pay my engineer $60 a month; 
that's $16.20 per horse-power per year; if you can furnish 
electric power for $15 per year perhaps we can trade." This 
theme, with variations, is familiar to anyone who has had 
practical experience in power transmission work, and although 
the more intelligent and able class of manufacturers are quite 
too keen not to see the facts when properly presented, a cer- 
tain amount of this ignorant short-sightedness is always met 
in investigating the power market. 

With a working 3^ear as above of 3,080 hours, the cost of 
steam power is actually very seldom as low as 1 cent per HP 
hour, and in units below 100 HP is not very often below 2 
cents. In units of less than 20 HP it is quite certain to be 5 
cents or more. These figures are based on continuous work- 
ing. If the use of power is intermittent, the cost per HP hour 



642 ELECTRIC TRANSMISSION OF POWER. 

is increased, by an uncertain but always large amount, depend- 
ing on the nature of the service. For highly intermittent 
service, gas engines are undoubtedly cheaper than steam, and 
in ordinary units the cost of operating these is seldom less than 
10 cents per HP hour of use. Used continuously at full load 
or thereabouts, the gas or petroleum engine is the most formid- 
able competitor of electric motors, since the actual cost of fuel 
is low — from 2 to 5 cents per HP hour — and the atten- 
dance required is trifling. Such engines, however, are high 
in cost and are inefficient at low loads, besides behig subject 
to relatively large depreciation. 

These peculiarities are well shown in a recent test of a 6 HP 
gas engine in which the following facts appeared: The cost of 
operation, including maintenance, was at full load 41 cents per 
hour, and at no load 20 cents per hour; the cost of gas being 
$1.70 per M feet. 

We may easily find from this the cost of power under given 
circumstances of use; $10 per HP per year may fairl)^ be 
charged up to interest and depreciation. Suppose, now, power 
is used for 10 hours per day 308 days in the year, the engine 
being fully loaded all the time. The cost can be made up as 
follows for 6 HP: ' 

3,080 hours fa) 41 cents = $1,262.80 

Interest and depreciation = 60.00 

Total cost = 11,322.80 

Cost per HP hour =7.15 cents, of which the interest and 
depreciation amounts to but 0.31 cents per HP hour. 

Second, suppose the engine is in full use 3 hours per day, and 
running idle the rest of the time, or is in equivalent partial use 
for 10 hours. We then have 

924 hours fa) 41 cents = $378.81 

2,156 " " 20 " = 431.20 

Interest and depreciation = 60.00 

= $870.01 

This is 12.08 cents per HP hour actually used, and is a fair 
type of present practice as gas engines are generally used. It 
will hold for the average engine used for small power purposes. 
In regular running such engines consume from 25 to 35 cubic 
feet of average illuminating gas per brake HP and, when run- 



THE COMMERCIAL PROBLEM. 



643 



ning light, take nearly half as much gas as at full load. In 
careful experimental running these results can be bettered 10 
to 20 per cent, but in regular work and with only ordinary care, 
the gas consumption given is correct. 

Petroleum engines give rather less fuel expense, but lose in 
extra care and repairs nearly or quite all the gain in fuel. 

These figures must not be understood as applying to large gas 
engines of 100 HP and upward, worked on cheap '^ producer" 
or fuel gas. It is reasonably certain that such engines give 
results better than any save the most economical steam en- 
gines, if worked at or near full load. The dubious point about 
such large gas engine plants is the maintenance, particularly 
in case a producer is installed. In the small sizes above con- 
sidered the gas engine is a considerably cheaper source of power 
than steam engines, probably by not less than 30 per cent. It 
must not be forgotten also that the cost of power from small 
gas engines is steadily being reduced owing to the great 
stimulus given to engine design and operation by the develop- 
ment of the automobile industry. 

In a general way we may summarize these facts regarding 
cost of power as follows, coal being taken at $3 per ton : 



Kind of Engine. 


Cost per HPH, 10- 
Hour Day, 
Fully Loaded. 


Cost per HPH, Inter- 
mittent Use, Partial 
Load. 


Large compound cond. . . . 
Simple, 100 HP and less . . . 

Gas, 20 50 HP 

Gas, small 

Steam, small 


8c. to Ic. 
1.5 " 2.5 
2.0 ♦' 4.0 
5. " 8.0 

7. " 12. 


Ic. to 1.5c. 

3. " 5. 

3. " 7. 
10. " 15. 
12. " 20. 



By small engines are meant those not over 15 to 20 HP, such 
as are used in large numbers for light manufacturing work. 
These figures are of course only approximate, and must be 
modified by the cost of fuel and labor in any particular locality. 

They take no account of the efficiency lost between the en- 
gine and its work, which has been already discussed in Chapter 
II, and which gives motor service some of its greatest commer- 
cial advantages. 

They show plainly, however, that electrical energy delivered 
to the consumer at 4 to 5 cents per kilowatt hour has the com- 



644 



ELECTRIC TRANSMISSION OF POWER. 



mercial advantage in small work of all kinds, and in competi- 
tion even with fairly large engines used at light load or inter- 
mittently. In addition there is, in favor of electricity, the 
generally considerable saving in waste power, and the greater 
cleanliness and convenience of the motor. At equal prices 
electric power will pretty effectively keep steam out of all new 
work, but the cost of changing from one motive power to the 
other demands some concessions on the part of electricity. 

This cost of change is rather uncertain, for not only do elec- 
tric motors vary very widely in price, owing to differences in 
size, speed, and construction, but the net value of engines and 
boilers replaced may vary from two-thirds to three-quarters 
of their cost down to little more than scrap. 

In both engines and motors the cost of the smaller sizes is 
disproportionately large, owing to the relatively large percen- 
tage of labor in their construction. Gas engines are even more 
expensive than a steam boiler and engine in ordinary sizes. 
In replacing engines by motors, the selling value of the former, 
including boilers, if steam is used, may be anything, say from 
$10 to $25 per HP, and the market is rather uncertain at best. 
A little time will generally effect a sale on tolerable terms. 

The following table gives the approximate cost of electric 
motors installed and ready to run, based on motors of ordinary 
speeds and voltages, with the usual accessories and with a 
moderate amount of wiring. No useful figures can be given 
on the cost of special installations with complex wiring. 



HP. 


Cost. 


1 


$ 75 to $ 125 


3 


150 " 250 


5 


200 " 275 


10 


300 " 450 


15 


350 " 450 


20 


400 " 600 


25 


500 " 700 


30 


600 " 800 


40 


700 "900 


50 


800 " 1,100 


75 


1,200 " 1,500 


100 


1,500 " 2,000 







THE COMMERCIAL PROBLEM. 645 

From this it appears that while large motors, 50 HP and 
upward, can generally be counted on at not over $20 per HP, 
the smaller sizes are much more costly. Below 20 HP the 
net cost of changing from steam or gas engines to motors is 
pretty certain to be $20 to $30 per HP. Taking interest and 
depreciation at 10 per cent, the annual charge amounts to $2 
or $3 per HP, which must be increased to $5 or $6 to cover 
maintenance and miscellaneous expenses. Hence, for steady 
use 10 hours per day, there should be charged to general cost 
about 0.2 cent per HP hour, which is equivalent to perhaps 
0.5 cent for intermittent use. 

In changing motive power, then, electric service must gen- 
erally be cheaper than what it replaces by about the amounts 
mentioned. 

As to the cost of furnishing electric power figures are a little 
deceptive, since from place to place the conditions vary. It is 
safe to allow about one KW at the station for one HP actually 
delivered and paid for. 

Now with steam for a motive power, the data already given 
for mechanical power can readily be reduced to kilowatt hours, 
assuming the dynamos to have as usual 92 to 95 per cent eflS- 
ciency at full load. But a steam station for power transmis- 
sion has the advantage of nearly or quite continuous running, 
thereby reducing general expenses, and besides, on a large scale, 
the load can be kept at an efficient point most of the time. 
In fact, in large railway power stations — the only steam-driven 
stations for power transmission on a large scale — the machines 
can be worked very efficiently most of the time, and power can 
be, and is, very cheaply produced. 

Fig. 317 shows graphically the approximate variation of 
total cost with output in well-designed power stations, the 
figures given being based on $3 per ton for coal and power 
delivered at the station bus bars. Anything under one cent 
per KW hour including interest, depreciation, superintendence, 
and general expense is good practice, even for a very large 
station. Steam is not likely to be often used as a motive power 
for power transmission work, except in working a very cheap 
coal supply. 

Dr. Emery has worked out at considerable length, the prob- 



646 



ELECTRIC TRANSMISSION OF POWER. 



lem of the cost of steam power on a very large scale and with 
the most economical modern machinery. He assumed a 
20,000 HP plant, worked 24 hours per day, on a variable load 
averaging 12,760 HP, 63.8 per cent of the maximum. This 
load factor is judiciously estimated and could certainly be 
realized in a plant of such size, employed in the general dis- 



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500 1000 1500 

Capacity of station in Kilowatts 

Fig. 317. 



2000 



tribution of power. Taking coal at one mill per pound, $2.24 
per long ton, and entering every item of expense, he found the 
total cost per HP per year to be $33.14. If the plant were 
established at the mouth of the coal mine, fuel should be ob- 
tained at not over one-third the above cost. This advantage 
would bring the cost per HP per year down to $24.89. Taking 
now 15,000 KW in dynamo capacity in large direct coupled 



THE COMMERCIAL PROBLEM. 647 

units, say five in number, the electrical plant would cost, 
installed with all needful accessories and ready to run, 
1200,000. Taking interest, taxes, and depreciation together 
at 10 per cent, which is enough, since a 3 per cent sinking 
fund would amply allow for depreciation; allowing $15,000 per 
year for additional labor and superintendence and $10,000 more 
for maintenance and miscellaneous expenses, brings the total 
annual charge for the electrical machinery to $45,000. Add- 
ing this to the steam power item and reducing the whole to 
cost per KW hour, assuming 94 per cent average dynamo effi- 
ciency, the total cost per KW hour delivered at the station 
switchboard becomes 0.436 cent. Working, then, on an im- 
mense scale from cheap coal, it is safe to say that less than half 
a cent per KW hour will deliver the energy to the bus bars. 

The next step is the cost of delivering it to the customer. 
This varies so greatly, according to circumstances, that an 
average is very hard to strike. A plant such as we are con- 
sidering will usually be installed only when the radius of dis- 
tribution is fairly long. Taking the transmission proper as 
50" miles, the line and right of way, using 30,000 volts, may be 
taken as about $25 per KW; the raising and reducing trans- 
formers with sub-station and equipment would cost perhaps 
$15 per KW, and the distributing circuits, with a fair propor- 
tion of large motors, about $10 per KW additional. The com- 
plete distributing system for 15,000 KW would then cost about 
$750,000. Figuring interest and depreciation roundly at 10 
per cent, the annual charge is $75,000. Add now $15,000 for 
labor in sub-station and distributing system, $10,000 for gen- 
eral administrative expense, and 5 per cent on the cost for 
maintenance and miscellaneous expenses, and we reach a total 
annual charge for distribution of $137,500. The average out- 
put being almost exactly 9,000 KW, the cost of distribution per 
KW hour is 0.174 cent. The actual cost of generating and dis- 
tributing the power then becomes 0.610 cent per KW hour. 

This is probably pretty nearly a minimum for distribution of 
power from coal mines. It supposes a very large plant in- 
stalled for cash and operated for profit. It makes no allow- 
ance for the floating of bonds at 60 to 80 cents on the dollar, 
the operations of a construction company, the purchase of 



648 ELECTRIC TRANSMISSION OF POWER. 

coal from the directors, the payment of big salaries to the pro- 
moters, or any of the allied devices well-known in financial 
circles. 

Under favorable circumstances a materially better result 
can be reached with hydraulic power. 

These figures mean that power could be sold at an average 
of 1 cent per KW hour at a good profit, aggregating for the 
plant in question more than a quarter of a million dollars per 
year. 

Only the largest plants, skilfully handled, can approach 
such figures for cost of power as have just been given. 

It should be possible, however, to bring the cost of distribu- 
tion per KW hour in a well-designed transmission plant of 
1,000 HP or more down to less than 0.5 cent per KW hour. 
Less than this may indeed be found in practice, while figures 
approaching 0.25 cent may be found in good central station 
working. 

The cost of producing power in steam-driven plants of vari- 
ous sizes has already been given; that in water-power plants is 
far less definite, but on the whole lower. In some hydraulic 
plants where development has been costly, the cost of water- 
power rises to $20 or $25 per net HP year, while on the 
other hand water-power has been leased at the canal for as 
little as $5 per year per hydraulic HP in the canal, equivalent 
to about $6.50 per available HP at the wheel shaft. The 
investment per effective HP at the wheel ranges from nearly 
$150 to as low as $30 or $40. This includes both the hydraulic 
rights and work and the wheels themselves. 

A typical estimate for a water-power plant under fairly fav- 
orable conditions, derived from actual practice, runs about as 
follows, for a 1,000 HP plant working at, say, 3,000 volts, so 

Hydraulic works $40,000 

• Wheels and fittings 12,500 

Power station 2, 500 

Pole line, 8 miles 4,000 

Transmission circuit 15,000 

Dynamos and equipment, 750 KW 15,000 

Transformers, 750 KW 7,500 

Distributing lines 15,000 

Miscellaneous 5,000 

Total $116,500 



THE COMMERCIAL PROBLEM, 649 

Operating expense: 

Interest and depreciation, 10 per cent $11,650 

Attendance at plant 4,000 

Linemen and team 2,000 

Office expense 3,500 

Rent, taxes, and incidentals 1,000 

Maintenance and supplies 4,000 

Total $26,150 

that there is no reducing sub-station, but only an ordinary 
distribution. 

The full capacity of the plant is about 750 KW. Supposing 
the plant to be worked somewhere near its capacity at maxi- 
mum load, and to be in operation on a mixed load 24 hours 
per day, we may estimate the daily output about as follows: 

KW KWH 

9 hours fa) 500 4,500 

5 " "250 1,250 

3 " " 100 300 

6 " " 50 300 

Total 6,350 

This should be taken for 300 days in the year. The other 
65 days, Sundays, holidays, and occasional periods of unusually 
small motor loads, it is not safe to count on more than 1,000 
KW hours per day. Taking account of stock, we have for the 
year, 

1,970,000 KWH, 

and the net cost per kilowatt hour becomes 1.33 cents. It is 
worth noting that the distribution of power for the day is taken 
from a transmission plant in actual operation. 

Of the above total cost, 0.47 cent is chargeable to distribution 
expenses and 0.86 to power production. Doubling the cost of 
the hydraulic works would raise the generating cost to 1.07 cents 
and the total cost to 1.54. 

It is evident in this case that power could be sold at 2 cents 
net per HPH with a good profit, assuming the smaller total 
cost, and at 2.5 cents, even with the greater hydraulic cost. 
Even if the total investment were as great as $250,000, the 
plant would pay fairly well at 3 cents per HPH. 

The fact is, hydraulic transmission plants generally will pay 
well if a good load can be obtained. The above example does 
not show a cheap plant nor a remarkable load factor. In 



650 ELECTRIC TRANSMISSION OF POWER. 

fact, the cost per KW in this case runs to about $155, while at 
present prices of material, many plants are installed at a 
considerably less figure even when as here the cost of the dis- 
tributing system is large. In really favorable cases the cost of 
power distributed will not exceed 1 cent per HP hour, and 
in comparatively few plants will it rise to 2 cents, unless the 
market for power is grossly overestimated. 

This is one of the commonest troubles with plants that do 
not pay well. A costly hydraulic development is undertaken, 
resulting in rendering available several times as much power 
as can be utilized; a portion of this is then transmitted and 
sold, but the plant is burdened with heavy initial expense, and 
struggles along as best it can. It is not safe to coimt on the 
stimulation of industrial growth by cheap power unless the 
situation is exceptionally fortunate, or cost of producing power 
is so small that the plant will pay tolerably well on the ex- 
isting market. 

A careful canvass for power is a necessary part of the pre- 
liminary work for a power transmission, and the more com- 
plete it can be made the better. Reference to the table of 
p. 643 shows that, at a selling rate of 2 to 4 cents per HP 
hour, the cost of power can be reduced for all small consumers 
and a good many rather large ones. If the cost of coal is 
high, $5 per ton or more, nearly all consumers will save by 
using electric power, while with favorable hydraulic conditions 
money can be saved by transmission even when replacing very 
cheap steam power. 

Take, for example, a large manufacturing plant requiring 
1,000 HP steadily, 12 hours a day. At a distance of, say, 8 
miles, is a hydraulic power that can give, say, 1,200 HP, and can 
be purchased and developed for $100,000. The cost of gener- 
ating and transmitting power will be about as follows: 

Hydraulic work 1100,000 

Wheels and fittings 15,000 

Power house 3,000 

Pole line 4,000 

Dynamos and equipment 20,000 

Transmission circuit 15,000 

Motors and equipment 15,000 

Miscellaneous 10,000 

Total 1182,000 



THE COMMERCIAL PROBLEM. 651 

and the operating expenses would be about as follows: 

Interest and depreciation $18,200 

Attendance at plant 2,600 

" " motors 1,800 

Other labor 1,000 

Maintenance, supplifes, etc 5,000 

Total 128^500 

This would furnish, taking the working year as 308 days, 
3,696,000 HP hours at a cost of 0.77 cent per HP hour. With 
a low cost of hydraulic development and a short line, say not 
over three miles, the above figures for cost could be brought 
down to about $130,000. Now, allowing 5 per cent for inter- 
est, and setting aside 3 per cent for sinking fund, which allows 
for complete replacement in less than 20 years, we may figure 
the annual cost of power again thus: 

Interest and sinking fund $10,400 

Attendance at plant 2,500 

" " motors 1,800 

Maintenance and incidentals 5,000 

Total $19,700 

This is $19.70 per HP year, or 0.53 cent per HP hour, or 
$15.80 per HP year omitting the sinking fund, which very sel- 
dom is allowed to creep into estimates on the cost of steam 
power. This is certainly cheaper than power can be generated 
by steam, save in very exceptional instances, provided proper 
account be taken of interest, depreciation, and repairs. As a 
matter of fact, the cost just given has been reached, in practice, 
in transmission work at moderate distances. On a larger scale, 
slightly better results can be attained. These figures take no 
account of the saving in actual power obtained by distributed 
motors, always an important matter in organizing a transmis- 
sion for manufacturing purposes. This can generally be 
counted on to make it possible to replace 1,000 HP in a steam 
engine by not over 750 HP in electric motors, with a corre- 
sponding reduction in the aggregate yearly cost of power. 

Speaking in a general way of costs at the present time (1906), 
dynamos and their equipment may safely be taken at $10 to 
$20 per kilowatt, raising and reducing transformers at from 
$4 to $8 per KW, line erected at from $10 to $30 per KW, 
water-wheels and governors at $10 to $20 per HP, and steam 



652 ELECTRIC TRANSMISSION OF POWER. 

plant, when used, at from $40 to $60 per net HP. Under fav- 
orable conditions the total cost per KW of capacity can be 
brought to $50 or $60 excluding all questions of steam plant 
and of hydraulic development. 

The line is always a rather uncertain item, on account of its 
variations in cost at different distances, and in meeting local 
conditions of distribution. The pole line itself will cost from 
$250 to $500 per mile, according to circumstances, but the 
copper must be figured separately, as already explained. 

No account is here taken of freaks in design — dynamos of 
special design for peculiar speeds or voltages, extraordinary 
line voltages, unusual frequencies, or eccentric methods of dis- 
tribution like the wholesome use of rotary converters and stor- 
age batteries. The figures are intended to represent ordinary 
good practice as it exists to-day. 

One of the nicest points in operating a transmission plant is 
the proper adjustment of the price of power to the existing 
market. It is no easy matter to strike the point between the 
cost of other power and the cost of generating and distributing 
electric power, which will give the maximum net profit. In 
general it is best to work entirely on a meter basis, for the 
customer then pays simply for what he uses, and the station 
manager knows the exact distribution of his output. 

The generating station or the sub-station should be equipped 
with a recording wattmeter that will show the actual output, 
and from this measurement much valuable information can be 
obtained. 

Knowing the investment and the approximate operating 
expense, it is easy to figure, as we have just done,the total cost 
of delivering energy per KW power at various outputs. This 
is the basis of operations. The next thing is to estimate as 
closely as possible the average local cost of power in units of 
various sizes. These two quantities form the possible limits 
of selling price. One must keep far enough above the first 
to insure a good profit, and enough below the second to cap- 
ture the business. It is convenient to plot these data as in- 
Fig. 318, which is based on the table of p. 643, and the plant 
discussed on p. 649. Curve 1 shows the effect of change in the 
annual output on the net cost per KWH. Curve 2 shows the 



THE COMMERCIAL PROBLEM. 



653 



approximate existing cost of steam or other power, the points 
from which the curve was drawn being shown by crosses. 
Curve 3 shows the same for intermittent loads, the points being 
indicated by circles. It is evident that for yearly outputs less 
than 1,000,000 KWH, the plant would be in bad shape to 
get business. At 2,000,000 KWH good profits are m sight, 



°^.^ 



H.P 



m 



M. 



M. 



500 1000 1500 2000 2500 3000 3500 4000 

annual output thousands of k.w. hours 
Fig. 318. 



while at 3,000,000, the electric plant can meet all cases at a 
profit. 

At the given output of 1,970,000 KWH, it would be possible 
to charge 2 cents per KWH as a minimum without losing busi- 
ness, while all the smaller customers could gain by changing 
to electric power at 4, 5, or 6 cents per KWH. 

When a few consumers are generating power at an unusually 
low figure, there is always the temptation to obtain them at a 



654 ELECTRIC TRANSMISSION OF POWER. 

special cut rate. As a rule, this is bad policy unless they are 
desirable for some particular reason aside from increase of out- 
put, for the moral effect of special low price contracts is always 
bad, and in the long run it is best to make standard rates and 
to adhere to them. 

The best prices can always of course be obtained from small 
consumers, and these are also specially desirable in that they 
tend to keep a uniform load on the system. Not only do 50 
10 HP motors yield ordinarily several times as much revenue 
as one 500 HP motor, but they will call for power very steadily 
all day long and keep the regulation excellent, while the large 
motor may be off and on in the most exasperating way and 
cause great annoyance at the time of the "lap load," when 
lights and motors are all in use. Large motors running inter- 
mittently are especially disadvantageous, for they do not 
greatly increase the aggregate station output and pay relatively 
little. 

In general, the best schedule of prices can be made up by 
starting with a rate arranged to get all the powers below, say, 
4 or 5 HP, and then for larger powers arranging a set of dis- 
counts from this initial rate. These discounts, however, 
should be based, not exclusively upon the size of the motors, 
but on the monthly KW hours recorded against them. In one 
respect, charging by wattmeter alone is at rather a disadvan- 
tage. A large motor running at variable load, and much of 
the time at light load, is far less desirable as a station load than 
a small and steadily running motor using the same number of 
KW hours monthly. The former demands far greater station 
capacity for the same earning power, and also inflicts a bad 
power factor upon the system at times of light load if the dis- 
tribution is by alternating current. It is not easy to avoid this 
difficulty, although various devices to that end have been intro- 
duced. In one large plant, recording ammeters are installed 
for each motor, and the largest demand for current lasting two 
minutes or more during a given month is made a factor in de- 
termining the price paid for that month's supply of power, so 
that large demands for station capacity must in part be paid 
for by the consumer. 

Another device for the same purpose is a combination of the 



THE COMMERCIAL PROBLEM. 655 

flat-rate and meter methods of charging. A fixed monthly 
charge per horse-power of the motor connected is made, and in 
addition the consumer pays for his energy by wattmeter, of 
course at a somewhat lower rate than in using the meter alone. 
A rough illustration of the effect is as follows. Suppose a flat 
charge of $1 per month per HP of the motor installed and a 
meter rate of 3 cents per KWH. One customer has a 10 HP 
motor worked steadily at full load 10 hours per day for 30 days. 
Another has a 50 HP motor which runs at full load for 2 hours 
per day. Each may, for example, use 3,000 KWH per month, 
and pay by meter $90 therefor; but the former pays a flat 
charge of $10, the latter one of $50, so that the monthly bill is 
in the former case $100, in the latter $140. The extra $40 may 
be regarded as the payment of rent for station capacity, and 
capacity of lines and transformers, to be held at the cus- 
tomer's call at all times. It is, in fact, a very genuine expense 
to the station. The whole question of equitable charging for 
current used for light and power is a very puzzling one. Tak- 
ing the country through, there has been a tendency for basic 
rates to cluster about 20 cents per KWH for lighting and 10 
cents per KWH for power. This difference has no logical 
reason for existence, and merely represents the natural ten- 
dency to get business by trying to keep below each consumer's 
supposed cost of production. The present tendency is to put 
current for lighting and power upon nearly the same basis, 
letting a sliding scale of discounts take care of the generally 
smaller output purchased by the lighting customer. These 
discounts vary greatly from place to place, but they generally 
run up to 50 to 70 per cent for large consumers, and are com- 
monly less for lighting than for power. On the whole, the 
simpler the system of rates and discounts, the better. 

It must not be forgotten that an electric supply company 
is a public service corporation doing business in virtue of fran- ' 
chise rights, and consequently it must tread softly and circum- 
spectly in its dealings with the public. Special contracts, save 
for an open and general reason, like the use of power during 
restricted hours, are from this point of view particularly to be 
avoided, and the whole rate system ought to be as open and 
above board as possible. 



656 



ELECTRIC TRANSMISSION OF POWER. 



A complicated system of discounts is not at all necessary to 
financial success, as witness the results obtained by the gas 
companies from a nearly flat rate. A simple and obvious dis- 
count scale generally is acceptable to the public, and the trouble 
really begins when one attempts to take account of differences 
in demand of the sort already mentioned. A minimum bill 
per lamp or HP installed plus a simple meter schedule can prob- 



4000 



8000 



KW. HR. PER MONTH 
12000 16000 20000 



24000 



S 



bs 




80 100 

horse power 

Fig. 319. 



ably take account of varying conditions as satisfactorily as any 
system yet devised. 

The exact form of the rate schedule can best be determined 
after looking over the local conditions. As an example of 
how the thing can be done, let us start with the data of Fig. 318 
on local costs of power. On the basis of curves 2 and 3, lay 
down a tentative curve of a sort fitted to get the business. 
Let this be, for example, curve 1 of Fig. 319. This naturally 
falls pretty near curve 2 of Fig. 318. Now at near full load a 
10 HP motor should take not far from 2,000 KWH per month. 
Hence, one can set down a rough scale of outputs correspond- 
ing to the horse-power of the several motor sizes. Of course 



THE COMMERCIAL PROBLEM. 657 

very few motors run fully loaded, and as they fall below full load 
their condition of economy approaches curve 3, Fig. 318, and 
their bills slip up along our present curve. By the curve a man 
with a 20 HP motor fully loaded will pay for 4,000 KWH a 
month at 6 cents, while if he consumes but 2,000 KWH he will 
pay for it at the rate of 6.7 cents per KWH. The man with a 100 
HP motor will get his power at 2.4 cents per KWH at full 
load, or at half load for 3.8 cents. These figures, although 
based on reasonable data, are, as compared with actual motor 
rates, rather high for the small motors and low for the 
large ones. The difference perhaps indicates that in general 
it has been found wise to encourage small motor business, as 
has already been indicated. 

Now to apply the combination of flat rate and meter to en. 
courage steady loads. A regular charge of $1 per HP o? 
motor per month will mean, on a basis of 200 KWH monthlj 
per HP, a fixed charge of 0.5 cent per KWH on the fully loadea 
motor. Therefore curve 1, Fig. 319, should be dropped down 
0.5 cent to get the new meter rates. Here is the chance for 
equalizing a bit, by dropping the upper end of the curve and 
letting its lower end alone. Curve 2 shows a curve thus low- 
ered. At about 50 HP the drop compensates for the fixed 
charge, and the total rates rise above that point, and below it, 
fall. Now for the 20 HP motor the monthly bill is $20 + 
4,000 KWH @ 5 cents = $220, and for the 100 HP motor 
$100 + 20,000 @ 2.3 cents = $560. The former pays $11 
per horse-power per month, the latter $5.60. At half input 
these figures rise to about $14 and $9.40 respectively. 

To simplify the discounts, curve 2 is commonly made up of 
a series of arbitrary steps such as are shown. They can be 
arranged to suit any case, the one shown being merely a simple 
example. Based upon it the discounts from the basic price of 
10 cents per KWH are: 

KWH Monthly Consumption ^6^06111*^ 

Below 400 

400- 1,000 20 

1,000- 3,000 40 

3,000- 5,000 50 

5,000- 9,000 60 

9,000-16,000 70 

16,000 and over 80 



658 ELECTRIC TRANSMISSION OF POWER. 

If there is an unusually good market for small motors, the steps 
can be arranged to favor them a bit, as, for instance, by giv- 
ing a 10 per cent discount from 200 to 400 KWH. It will 
be seen that the whole scheme is frankly empirical, although 
based on premises which are not without reason. Some dis- 
count schedules are far more complex than that shown, while 
others are rather simpler. The prices given here are fairly 
high save on the large motors. Many plants give an extra 
discount of 5 or 10 per cent for prompt settlement. In selling 
current for lighting, the discounts are generally less variable 
with the consumption than here shown, and a flat service rate 
in addition is of rather dubious expediency considering the 
policy of the gas companies. The discount schedule here 
given would do very well for the lighting output as well as 
for the motor load. 

Charging by a recording ammeter instead of a wattmeter 
will reach the users of motors that injure the power factor of 
the system, and, combined with the flat rate just mentioned, 
Avould probably give a really fairer system of payment for the 
customer's demand upon the station than either of the schemes 
just described, but the wattmeter is so generally used and 
understood that it can hardly be escaped. 

Methods of selling and charging, however, must be modified 
to suit local conditions and customs. Each community has 
peculiarities of its own that must be studied and reached. 
Sometimes a flat rate, objectionable as it often is, will secure 
a more remunerative business than any system of metering, 
while elsewhere a meter system, however intricate, may work 
better than a flat rate. As a rule, however, metering is the 
best method of charging for all parties. 

A water-power transmission plant has the peculiarity when, 
as usual, the water is owned outright, of showing a nearly 
constant operating expense, irrespective of output. Hence, 
after the receipts exceed this expense, all additional load, at 
any price, means profit. But it means profit precisely in pro- 
portion to its price, so that taking on large consumers at a 
very low price is usually bad policy, it being better to encour- 
age small consumers by giving what is to them a very reason- 
able figure. 



THE COMMERCIAL PROBLEM. 659 

After the maximum output comes near to the capacity of 
the plant, the total yearly output for the given plant is diffi- 
cult to increase. Hence, it is desirable persistently to culti- 
vate the use of power at such times as will not increase the 
maximum load. This can best be done by offering liberal dis- 
counts for power used only between, say, 8 p.m. and 8 a.m. 
There is at best rather a small amount of this, and it is all 
worth getting even at a low rate. After getting all the avail- 
able night power, the next step should be to get whatever day 
business is possible for hours restricted to the period prior to 
the beginning of the peak, say at 4 p.m., again at special dis- 
counts. Now and then a customer can be picked up on this 
basis to the great advantage of the station. 

In stations using rented water-power at a fixed price per 
HP, or employing steam, the operating expense is of course 
variable, and this variation will influence greatly the adjust- 
ment of prices, although the general principles are unchanged. 

Experience has now shoT\m that electric power transmission 
may generally be made a profitable enterprise. 

If a transmission is planned and executed on sound business 
])rinciples and with ordinary forethought, it is well-nigh cer- 
tain to be a permanent and profitable investment. 

Failure is generally chargeable to attempts to work with 
altogether insufficient capital, leading to ruinous actual rates 
of interest; the purchase of material at extortionate prices 
due to various forms of credit; and huge commissions to 
promoters. 

Organized in such wise, almost any enterprise becomes 
merely speculative, and its failure should produce neither sur- 
prise nor sympathy, for such a course is the broad highway that 
leads straight into the ever ready clutches of a receiver. Hon- 
esty is the best policy in power transmission, as elsewhere. 



CHAPTER XVII. 

THE MEASUREMENT OF ELECTRICAL ENERGY. 

The basic fact regarding the measurement of electrical 
power is the stress between a magnetic field and a coil carry- 
ing a current. Obviously such a coil produces of itself a mag- 
netic field, but it is the proportionality of this field to the cur- 
rent rather than its mere existence that gives it importance in 
measuring instruments. 

The fundamental measurements which have to be made in 
ordinary practical engineering are three — current, electro- 
motive force, and electrical energy, which is their co-directed 
product. In continuous current work, while mere readings 
of the first two give the energy as their numerical product, it is 
generally desirable to have instruments which measure energy 
directly and which integrate a varying output continuously, 
so that one may at all times keep track of the output of the 
station, a single circuit, or the energy supplied to a single cus- 
tomer. In alternating current work a wattmeter is doubly 
necessary, first because the product of volts and amperes does 
not give the real energy, but the apparent energy, as has 
already been explained; and, second, because the true energy 
divided by the apparent energy equals the power factor, which 
should be looked after very carefully in an alternating station. 

Any effect of electric current which is proportional to or 
simply related to that current may obviously be used for its 
measurement, and in laboratory measurements instruments 
based on almost every imaginable property of electric current 
have been used with more or less success. But for e very-day, 
practical purposes instruments must possess qualities not so 
important in the laboratory, so that the possible types of m^eas- 
uring instrument have simmered down to a very few, with 
respect to the principles concerned. 

So far as continuous currents are involved, nearly all prac- 
tical instruments are electro-magnetic, as has already been 

660 



THE MEASUREMENT OF ELECTRICAL ENERGY. 661 

indicated — almost the sole exception being the Edison chemi- 
cal meter, which need not here be described, since it is passing 
rapidly out of use. 

The simplest electrical measuring instrument is the ammeter, 
designed for the practical measurement of current strength. 
In its commonest forms, as used for continuous current, it 
consists of a fixed coil of wire carrying the current to be meas- 
ured and a pivoted magnetic core, to which is attached a pointer 
sweeping over a fixed scale. The force on this core varies with 
the current, and is resisted by some opposing force that brings 
the pointer into a new point of equilibrium for each value of 
the current. Sometimes this opposing force is the magnetic 
field of the earth, as in the ordinary laboratory galvanometer, 




Fig. 320. 



but in practical instruments it is generally gravity, a spring, 
or a relatively powerful permanent magnet. 

Most of the numerous varieties of ammeter have been pro- 
duced in the effort to secure a permanent and constant control- 
ling force, and uniformity of scale; that is, such an arrangement 
of parts as will make the angular deflection of the pointer 
directly proportional to the amperes flowing through the coil. 
The result has been all sorts of curious arrangements of the 
coils and the moving armature with respect to each other, and 
the upshot of the matter generally is that the scale has to be 
hand-calibrated for each instrument, the divisions of the scale 
being fairly uniform through the parts of the scale most often 
used, but varying somewhat near its ends. In first-class mod- 
em instruments, a remarkably even scale is attained. Fig. 320 
is a good example taken from the scale of a regular station 
ammeter. Gravity is far and away the niost reliable control- 
ling force, but it is also highly inconvenient hi instruments 
intended for portable use or for a wide range of action while 



662 



ELECTRIC TRANSMISSION OF POWER. 



still preserving small inertia in the moving parts, so that springs 
or permanent magnetic fields form the main reliance in practice. 
In some admirable instruments the well-known principle of the 
D'Afsonval galvanometer is employed. In this instrument, of 
which a familiar laboratory type is shown in Fig. 321, a light 




Fig. 321. 



movable coil is suspended between the poles of a very powerful 
permanent magnet, shown in the cut as built up in circular 
form. Current traversing the coil through the suspension 
wires sets up a field, which, reacting with the magnet, pro- 
duces a powerful deflecting force on the coil, controlled by the 
torsion suspension. In commercial instruments the suspen- 
sion is replaced by jeweled bearings, and the current is led in 



THE MEASUREMENT OF ELECTRICAL ENERGY. 663 

through the controlling hair springs or by very flexible leads. 
The resulting instrument is very sensitive and accurate for the 
measurement of small currents or known fractions shunted 
from larger ones. The famous Weston direct current instru- 
ments, together with others less well known, are constructed 
along this general line. 

The sources of error even in the best commercial ammeters 
are many. Permanent magnets and springs do not always 
hold their strength precisely, jeweled bearings wear, and break 
if the instruments are roughly handled, pointers get bent, 
dust sometimes gets in, and on these accidental errors are 
superposed those due to errors in scale and calibration. 
Nevertheless, the best station and portable ammeters possess 
and maintain a very commendable degree of accuracy. When 
carefully handled and used well within their working range 
they can be trusted to within about one or two per cent. If 
of the highest grade and frequently verified, they can be relied 
on in the best part of the scale down to, say, half the above 
amount, and under circumstances exceptionally favorable will 
do even a little better. In laboratory work, where they are 
merely used as working instruments and often checked, it is 
possible to nurse them into still higher accuracy, but one cannot 
depend upon it for long at a time under commercial condi- 
tions. For relative measurements only, made within a short 
time, high-grade ammeters are very accurate, but the hints 
already given should make it clear that when in regular use one 
must not expect to use them for absolute measurements with 
a great degree of precision. The cheaper class of instruments 
is likely to show double the errors just noted. 

For the measurement of alternating currents only a few of 
the types of ammeter used for continuous current are appli- 
cable. Hysteresis in the iron parts and reactance in the coils 
are likely to incapacitate them, but some of the forms can 
readily be modified to give good results, and certain others are 
specially suited to alternating currents. In this work a new 
class, having a fixed field coil reacting on an armature 
coil capable of rotation, and spring controlled, has been 
made generally useful. These instruments are derived 
from the laboratory electro-dynamometer much as those pre- 



664 ELECTRIC TRANSMISSION OF POWER. 

viously mentioned are derived from the D' Arson val galvano- 
meter, and are capable of similar precision in practice. On 
account of their extremely small reactance, hot wire instru- 
ments have retained still some slight measure of their one-time 
popularity. In this class of instruments the current is passed 
through a fine suspended wire of rather large resistance, which 
is thereby heated and expands, carrying with it the pointer to 
which it is attached, usually by means of multiplying gear. 
Such instruments require correction for the temperature of the 
air, but are capable of very good accuracy if carefully handled. 
They are ''dead beat," i.e., the pointer comes to rest without 
oscillation, a very useful property, which is secured to a cer- 
tain extent in most instruments by various damping devices. 
Instruments having a powerful permanent magnet often are 
supplied with a copper damping vane, which checks oscillations 
by virtue of the eddy currents stirred up in it by the magnet ; 
and sometimes air vanes in a close-fitting recess or light me- 
chanical stops, which can be brought up against the moving 
parts, are used for this purpose. For high-voltage generators 
the current for the instruments is derived from a current trans- 
former, as the instruments themselves are difficult properly 
to insulate for more than 2,000 to 2,500 volts. They are used 
with instruments graduated to show the primary current, a 
known fraction of which is actually derived from the secondary. 
Such a current transformer for moderate currents is shown in 
Fig. 322. It is designed merely to furnish current for the 
ammeter and wattmeters. 

Voltmeters for measuring the electromotive force are in all 
general points constructed precisely like ammeters, save that 
the working coil, whether fixed or movable, is wound with very 
fine wire in many turns, so as to be adapted to work with very 
small currents, and usually has in series with it a resistance of 
several thousand ohms. Voltmeters are in fact ammeters 
having so much resistance permanently in circuit that the 
current which flows through them is substantially proportional 
to the voltage across the points to which the instrument is con- 
nected, irrespective of other resistances which may casually be 
in circuit. Only in rare instances, as sometimes in incandes- 
cent lamp testing, is the current taken by the voltmeter a 



THE MEASUREMENT OF ELECTRICAL ENERGY. 665 

source of perceptible error, and in such cases it is readily al- 
lowed for. Voltmeters are more difficult to construct than 
ordinary ammeters, owing to the fine wire windings and the 
high resistance, and are generally rather more expensive. 

They are capable of just about the same degree of precision 
as ammeters, being subject to about the same sources of error. 
When used for alternating current, the large auxiliary resist- 
ance is wound non-inductively, and the working coil is propor- 
tioned for as low reactance as may be possible with the required 
sensitiveness. For measuring very high alternating voltages, 
a ''potential transformer,'^ shown in Fig. 323, as adapted for 
high-voltage transmission systems, is used. These transformers 




Fig. 322. 

have usually a capacity of from 50 to 250 watts, and are used 
for the instruments only. They are wound with an accurately 
known ratio of transformation, receive the high-pressure current, 
and deliver it to the voltmeter at a more reasonable voltage. In 
dealing with continuous currents the problem is more difficult. 
Sometimes a very sensitive voltmeter is provided with a sep- 
arate high-resistance box, reducing the scale readings to some 
convenient fraction of their real value, so that the instrument is 
used with a constant multiplier to transform its readings to 
the corresponding voltage. This is a useful device for obtain- 
ing the voltage of arc circuits and the like. 

In default of high- voltage instruments, a rack of incandescent 
lamps may be wired in series and voltmeter readings taken 
across a known fraction of the total resistance thus inserted; 



666 



ELECTRIC TRANSMISSION OF POWER. 



250- volt lamps in sufficient number not to be brought up to full 
candle-power are convenient for this purpose, and the volt- 
meter should be of so high resistance that its presence as a shunt 
around part of the lamps will not introduce material error. 

A generating station should be liberally equipped with am- 
meters and voltmeters. Besides the ordinary switchboard 
instruments, usually an ammeter for each machine and each 




Fig. 32S. 



feeder, it is desirable to have several spare instruments which 
can be temporarily put in for testing purposes. Station in- 
struments should have large, clearly divided scales and con- 
spicuous pointer, so that the readings can be seen at a distance 
from the switchboard. The large illuminated dial instruments 
are excellent for the principal circuits, and the main station 
voltmeters may well be of similar type. To save space such 



THE MEASUREMENT OF ELECTRICAL ENERGY. 667 

instruments are very commonly made with scales arranged 
edgewise as in the station voltmeter shown in Fig. 324. 

Voltmeters are ordinarily not numerous in a station, and 
are usually arranged with changeable connections, so that 
they may be plugged in on any circuit and mounted on 
swinging brackets so as to be readily visible from various 
directions. There should, however, always be at least one 
conspicuous voltmeter permanently connected to show the 
working pressure on the main circuits. In polyphase work, 
this should be capable of being plugged in on each phase, 
although it is preferable to have a voltmeter permanently 
on each phase in large transmission work. At least two other 




Fig. 324. 



voltmeters should be available for connection to such circuits 
as may be desirable, in testing circuits, parallelizing machines, 
and the like. These ought to be small switchboard instru- 
ments of the highest grade, mounted side by side to enable 
comparative readings to be readily made. As potential 
transformers for high voltage are decidedly costly, a simple 
and safe arrangement for plugging in the primary side of such 
a transformer on any high voltage connection is much to be 
desired. A duplicate or spare potential transformer should 
always be kept in stock, since it is most inconvenient to have 
a voltmeter thrown out of action. In stations having high 
voltage generators it is sometimes practicable to connect for 
the voltmeters around a single fixed armature coil in each 
generator, which much simplifies the transforming arrange- 
ments. 



668 ELECTRIC TRANSMISSION OF POWER. 

Indicating wattmeters reading the output directly are not in 
by any means as general use as ammeters and voltmeters, 
but are highly desirable in portable form for motor and lamp 
testing, and should be seen upon the switchboard far oftener 
than they are. These instruments follow the same general 
line of design as ammeters and voltmeters, but are provided 
with two working coils or sets of coils. One takes the current 
of the line on which the output is to be measured either directly 
or through a current transformer, and the other is a voltmeter 
coil suspended so as to turn in the field due to the current coil. 
The torque produced obviously depends on the product of 
the two fields due to the coils respectively, which is propor- 
tional to the energy delivered. If the two fields are in the 
same phase, as in continuous current practice, or at times 
of unity power factor in alternating circuits, the numerical 
product of the two field strengths is proportional to the total 
energy; but if there is difference of phase, then the co-directed 
components of the two fields are proportional to the energy. 
The controlling and damping forces are like those in ammeters 
and voltmeters, and the wattmeters differ little from them 
in general arrangement save for having two sets of terminals, 
one for current and the other for potential, and in the gradua- 
ation of the scale. An indicating wattmeter is at times 
a valuable addition to a generator or feeder panel, but it is 
not necessary in the same sense as ammeter or wattmeter. 

A well-equipped station should also have two or three such 
instruments in portable form, one for the testing of incandes- 
cent lamps and such small outputs, and others capable of 
taking the output delivered to the ordinary sizes of motors 
and recording wattmeters. It should also have a set of por- 
table ammeters capable of reading the ordinary range of cus- 
tomer's currents without getting off the good working portions 
of their respective scales. For instance, if one ammeter will 
read with good accuracy from 1 to 10 amperes, the next might 
go effectively from 5 to 25 amperes, and the next from 20 
to 60. 

Of portable voltmeters there should be enough to measure 
accurately the voltages used for the distribution, and a por- 
table potential transformer to enable primary voltages to be 



THE MEASUREMENT OF ELECTRICAL ENERGY. 669 

dealt with in an alternating system. It is desirable to have 
a pair of exactly similar voltmeters to use in simultaneous 
readings for drop, and to check each other and the station 
instruments. 

Another form of voltmeter regarded by the author as a 
necessity in every power transmission plant is a recording 
instrument keeping a continuous permanent record of the 
voltage and its variations. Such a record is shown reduced 




Fig. 325. 



in Fig. 130, page 234. The Bristol voltmeter is the form of 
instrument most commonly seen, and is shown in Fig. 325. 
It is merely a strongly made voltmeter with a long pointer 
carrying a pen, and swinging from centre to circumference of 
a paper disk driven by a clock and ruled in circles for the volts 
and radially for time. A variable resistance permits it to be 
accurately adjusted to agree with a standard voltmeter, and 
when carefully managed it is quite reliable. As a check on 
the operation of the station and for reference in case of dispute 
it is invaluable, since it shows every variation of voltage, 



670 



ELECTRIC TRANSMISSION OF POWER. 



and the time at which it occurred. In using it the pen should 
be kept clean and smooth running, bearing just heavily enough 
to leave a sharp, thin line, and the clock should be very care- 
fully adjusted to keep correct time. The chart should be 
changed at the same time each day and put on so as to record 
the correct time. 

Recording ammeters and steam gauges are made upon a 
similar principle, but for power transmission plants the volt- 
meter is the most important instrument. Installed in the 




Fig. 326. 



generating ' station it keeps accurate record of the regulation, 
and in the sub-station it serves a similar purpose. 

An instrument sometimes used of late is a frequency meter, 
showing on its dial the periodicity at any time just as an 
ammeter shows the current. Its principle is very simple. 
Any voltmeter having some considerable reactance will change 
its reading with change of frequency. If furnished with a 
scale empirically graduated for different frequencies, it be- 
comes a frequency meter, and if installed where the voltage 
is fairly constant and designed so as to be hypersensitive to 
changes of frequency, it serves a useful purpose in telling 
whether the machines are at the exact speed intended. In 



THE MEASUREMENT OF ELECTRICAL ENERGY. 671 

fact it could in any given situation be graduated for speed as 
well as for frequency. 

Occasionally recording wattmeters, similar to the recording 
voltmeters already described; are used ; but it is difficult to get 
accurate readings over a wide enough range to be of much 
use, and the more usual instrument is the integrating watt- 
meter, sometimes referred to as recording, which registers the 
output in watt hours continuously. Instruments of this class 
are used both to register the energy supplied to customers and 
to take account of the energy generated. Daily readings of 
the switchboard mstruments give by difference the daily 
output in KW hours, and in steam driven stations are most 
important in keeping record of the station efficiency and its 
variations. It is needless to say that instruments used for 
this purpose should be kept in especially careful calibration 
since errors in the whole output are dealt with. Even in 
hydraulic stations they give a useful check on station opera- 
tion and on the energy sold. 

Integrating wattmeters are essentially motors whose speed 
is proportional to the output. Like indicating wattmeters 
they produce a torque due to the co-action of current and 
potential coils, and the armatures revolving under this stress 
are furnished with an automatic drag due to a disk revolving 
between magnet poles or to air varies, so that the speed shall 
be proportional to the output on the circuit in watts. Prob- 
ably in principle the simplest of these instruments is the widely 
known Thomson recording wattmeter. Fig. 326 shows the 
general appearance of this meter with the cover removed, and 
Fig. 327 gives its connections in the ordinary two-wire form. 
Essentially it consists of the following parts: a pair of field 
coils of thick wire, in series with the load; an armature, drum 
wound, of very fine wire, in series with a large resistance and 
placed across the mains; and a copper disk on the armature 
shaft revolving between the poles of three drag magnets. 
The fields and armature are entirely without iron, the arma- 
ture shaft rests on a sapphire or diamond jewel bearing, and 
its upper end carries a worm to drive the recording gear. 

The commutator is of silver of which the oxide is a fair 
conductor so that the commutator does not easily get out of 



672 



ELECTRIC TRANSMISSION OF POWER. 



condition, and current is taken to it by slender copper brushes 
restingtangentiallyupon it. The drag magnets are artificially 
aged, so that they remain very permanent and are adjustable 
to regulate the meter, if necessary. The resistance of the 
potential circuit is several thousand ohms, and the loss of 
energy in the meter at full load does not often exceed 5 to 10 
watts. As the static friction of the armature is considerably 
greater than the running friction, the ''shunt" in the poten- 
tial circuit is made part of the field, so as to help the meter 
in starting. Doubling the current evidently doubles the 
torque in such a motor meter, but since the work done in eddy 




Fig. 327. 



currents in the drag increases as the square of the speed, the 
armature will run at a speed directly proportional to the 
energy, which is the speed desired. 

In point of fact, such meters are capable of giving very great 
accuracy — within two per cent under ordinary good commer- 
cial conditions, and very uniform results under different con- 
ditions of load. 

Such meters are suited for use on both continuous and alter- 
nating circuits, and are remarkably reliable in their indications 
under all sorts of conditions, save with very low power factors. 
Another and very beautiful group of meters is designed espe- 
cially for use on alternating circuits only, and follows the prin- 
ciple of the induction motor, just as the Thomson meter is a 



THE MEASUREMENT OF ELECTRICAL ENERGY. 673 

commutating motor. The pioneer of this class was the famous 
Shallenberger meter, an ampere-hour meter, Avhich has been 
very widely used and would be extremely useful where am- 
peres rather than watts are to be measured, although now prac- 
tically abandoned. 

A fair type of the induction wattmeter is shown in Fig. 328, 
the Scheefer meter, one of the earliest of the class, although 
here shown in a recent form. It consists of a finely laminated 




Fig. 328. 



field magnet energized by a current coil and a potential coil; 
an aluminium disk armature, and the magnetic drag which hag 
come to be generally used in meters. A priori one would sup- 
pose that so simple a structure could hardly be made to give 
an armature speed proportional to the energy in the circuit; 
and in fact it takes great -finesse to design it so as to accomplish 
this result, but it can be successfully done, and meters of this 
class turned out by various manufacturers are capable of doing 
very accurate work. 

As a class they develop very small torque, but in part make 
up for this failing by the very small weight of armature and 



674 ELECTRIC TRANSMISSION OF POWER. 

shaft. The speed is seldom accurately proportional to the 
energy over a very wide range of load, but day in and day out 
the small errors generally tend to balance each other, so that 
the total reading at the end of a month varies but little from 
the facts. The induction meters are liable to material errors 
in case of large change of voltage, power factor, or frequency, 
but within the range of these factors in ordinary service they 
do sufficiently accurate work for all commercial purposes, 
and the best of them are substantially as accurate as the com- 
mutating meters. 

All types of meters are made suitable for switchboard work 
in measuring large outputs, and in alternating stations can be 
fitted for use on primary circuits, although this is seldom nec- 
essary, and should not be attempted at any but moderate vol- 
tages without the use of transforming apparatus for the meter. 
Most switchboard meters for such work as power transmission 
are of special designs, modified for the particular work in hand. 

Monophase alternating circuits and continuous current cir- 
cuits are measured in the most direct wa}^ possible, the am- 
meters being put in the mains, and the voltmeters across them, 
through a potential transformer if need be, as it is somewhat 
troublesome to wind voltmeters for use directly upon circuits 
above 2,000 to 2,500 volts. 

Wattmeters are connected to such circuits in a similar 
straightforward way, shown for continuous or secondary alter- 
nating current in Fig. 327 and for primary alternating circuits 
in Fig. 329. In these and other cuts of wattmeter connections 
the circuits of the Thomson meter are show^n, but they must 
be regarded as merely typical, since in using other meters the 
arrangement of circuits follows the same principle, the field or 
current coil being put in the mains and the armature or poten- 
tial circuit across them. The former is wound with coarse wire 
or copper strips, the latter with very fine wire, so that they can 
very easily be told apart even at a casual inspection. 

Ordinary two-phase circuits are measured in a precisely simi- 
lar fashion, each pair of phase-wires being treated as a separate 
circuit and supplied with its own instruments. The metering 
likewise, whether of primary or secondary circuits, is gen- 
erally accomplished by the use of two wattmeters, each con- 



THE MEASUREMENT OF ELECTRICAL ENERGY. 675 



nected to its own pair of mains. In case of motors in which 
the two phases may be regarded as substantially balanced and 
equal, it is only necessary to put the instruments in one of the 
phases and to multiply the readings of energy by 2 to get the 
total input. If the two-phase circuits are unbalanced, two sets 
of instruments are absolutely necessary for a simultaneous read- 
ing on both phases, unless some form of combined instrument 
takes their place. 

With three-phase circuits the case is rather more compli- 
cated. The simplest to manage is a star-connected three- 
phase balanced circuit, as found in some motors. Here the 




Fig. 329. 

ammeter or current coil of the wattmeter goes directly into 
one lead, and the voltmeter or potential coil of the wattmeter 
is connected between that lead and the neutral point of the star. 
The instrument then gives correctly one-third of the energy. 
Therefore, the wattmeter reading multiplied by 3 gives the 
energy on the circuit. On some of the early three-phase motors 
of which the primaries were star-wound, an extra lead was 
brought from the neutral point to the connection board to 
facilitate measurements. On a circuit mainly of motors fair 
balance usually exists. 

If the circuit is balanced it is not necessary that a star con- 
nection at the generator or transformers should either be easily 
accessible or exist in order to use the method of measure- 
ment just described. For if the circuit is balanced the am- 



676 



ELECTRIC TRANSMISSION OF POWER. 



meter or current coil of the wattmeter may be put in a lead, 
and the voltmeter or potential coil of the wattmeter be con- 
nected between the same lead and the neutral point formed 
by three equal high resistances connected to the three leads 
respectively, and with their three free ends brought to a com- 
mon junction. Such an artificial neutral is very commonly 
used in connecting wattmeters on the secondary circuits for 
motors, and may be applied to primary circuits as well. The 
writer has sometimes constructed such a neutral by connect- 
ing three strings of incandescent lamps to the three leads and 




Fig. 330. 



to a common junction. Then connecting the potential coil of 
a wattmeter around one lamp and its current coil in the lead to 
which the string containing this lamp ran, it became possible 
to make a closely approximate measurement of the primary 
energy with only an ordinary 110 voltmeter and such applir 
ances as can be picked up around any station. This device 
of an artificial neutral as applied to secondary circuits is well 
shown in Fig. 330. 

The measurement of energy on an unbalanced three-phase 
circuit is a very different proposition. Of course three watt- 



THE MEASUREMENT OF ELECTRICAL ENERGY. 611 

meters with their three potential coils respectively in the three 
branches of a star-connected resistance, such as has just been 
shown, would do the work, but at a very undesirable cost and 
complication. 

If, however, two wattmeters are used with their current coils 
in two phase-wires respectively, and their potential coils re- 
spectively between their own phase-wires and the remaining 
wire of the three, the sum of the readings of these two meters 
records correctly the total energy of the circuit. Such an 
arrangement of meters is shown in Fig. 331, as commonly 




Fig. 331. 



applied to three-phase secondary circuits. A precisely simi- 
lar arrangement with the addition of potential and current 
transformers is used for primary circuits. 

In a similar connection two indicating wattmeters will give 
the energy of the circuit at any moment. An indicating 
wattmeter with its current coil in one phase-wire of a three- 
phase system, will give three diverse readings according as its 
potential coil is connected between its own wire and each 
of the other phase-wires, or finally across the two other phase- 
wires. The latter reading is dependent on the angle of lag, 
being zero for unity power factor, and a wattmeter so con- 
nected can be used as a phase meter, while the other read- 
ings will be respectively increased and diminished to an amount 
dependent on the lag. 



678 . ELECTRIC TRANSMISSION OF POWER. 

Many attempts have been made to combine the two watt- 
meters necessary to measure correctly the energy on aa un- 
balanced three-phase circuit into a single instrument, and 
recently with considerable success. Fig. 332 shows a com- 
bined induction wattmeter, for two- or three-phase circuits, 
balacend or unbalanced, and Fig. 333 its connections with 
current and potential transformers as viewed from the front 
when used on a three-phase, or three-wire two-phase circuit. 
If the three-phase circuit to be measured be a balanced one, 
such a composite wattmeter need merely have a current coil 
connected in either lead and a pair of potential coils connected 
from this to the adjacent leads respectively. In testing motors 




Fig. 332. 



one can readily get the same result if the load be uniform, 
by using an indicating wattmeter with one lead connected 
through its current coil and then switching the potential con- 
nection successively to the adjacent leads, and adding the two 
readings. Instruments for unbalanced circuits should have 
two currents and two potential coils, as already indicated. 

It should be noted that in balanced mesh-connected circuits 
one can measure the energy correctly by putting the current 
coil of the wattmeter into one side of the mesh inside the joint 
connection to the lead, and the potential coil across the same 
side of the mesh. This gives a reading of one-third the total 
energy. Ammeter and voltmeter similarly connected give 
readings showing one-third the apparent watts. 

In ordinary three-phase distributing systems the actual 



THE MEASUREMENT OF ELECTRICAL ENERGY. 679 

metering is much simpler than would appear at first sight. 
Motors are provided with a single meter, usually connected as 
shown in Fig. 330. Much of the lighting is from a pair of 
phase-wires or from one phase-wire and the neutral, in which 
case the secondary service is a simple two-wire distribution 
measured like any other monophase system. In cases where 
all three wires are taken into the same service, the energy 
can be measured by two meters, as shown in Fig. 331, or by 
a meter like Fig. 332. 

The induction type of meter is sometimes liable to consider- 




FlG. 333. 



able errors on motor circuits where the power factor is subject 
to large variations, and should therefore be used with caution. 
Before purchasing meters it is advisable to ascertain by actual 
tests how they will perform on circuits of varying power factor. 
In the case of large station meters especially in polyphase 
stations, it is necessary as already indicated to take especial 
precautions. In the first place, the meter in such cases has 
a large constant, since it is operated from current and poten- 
tial transformers, each of which transforms down to the meter. 
Assuming that the transformation ratios of these transformers 
are correct, there are still some residual errors that must be 



680 - ELECTRIC TRANSMISSION OF POWER. 

looked out for. Unless the potential transformer has a negli- 
gible drop of potential under load, which is never really the case, 
the voltage supplied by it to the wattmeter will vary slightly 
w^ith the load upon the transformer. Hence, for station watt- 
meters, separate transformers should be used if high precision 
is expected. 

Second, there will be some variation of the phase displace- 
ment between primary and secondary E. M. F. due to the trans- 
former reactance, which in a wattmeter is combined with 
another slight phase shift between primary and secondary 
current in the current transformers due to the magnetizing 
current required for the transformer core, and varying with 
the power factor. There may also be slight error from the 
wiring loss between the transformer and meter. None of these 
items would be of practical account in ordinary meter work, 
but in a station or other large meter they may be significant. 
They are of a combined magnitude which may be two or three 
per cent, in other words larger than any ordinary errors in 
the meter and may at times be additive with respect to these, 
so that they must be taken account of. Special transformers 
for the main wattmeter, and careful meter calibration for an 
average value of the power factor, will go far toward reducing 
such errors to a negligible amount. 

Stray magnetic fields about the switchboard may also cause 
very appreciable instrument errors, and should be looked out 
for assiduously. 

Meters should be installed where thej^ will be free from vibra- 
tion, extreme heat, and dampness, chemical fumes, and dust. 
To a less extent the same rules apply to other instruments, but 
meters with their constantly moving parts and very light 
torque should be looked after with particular care. They 
should be thoroughly inspected every few months, and at less 
frequent intervals should be carefully tested in situ, which can 
very readily be done by the aid of an indicating wattmeter 
connected to the same load. The following formula serves for 
this test : 

3600 X Constant of meter (if any) 

Watts in use ~" 

seconds per revolution of armature. 



THE MEASUREMENT OF ELECTRICAL ENERGY. 681 

Nearly all meters use the magnetic drag, and a light mark 
near the periphery of the meter disk timed for a few revolu- 
tions with a stop watch, gives the right hand side of the equa- 
tion, while the watts input is checked by the indicating watt- 
meter. The constant of the meter by which its reading must 
be multiplied to give the true energy recorded is nearly always 
plainly marked as an integral number upon the meter. If a 
meter shows material error it can be brought to the correct 
rate by slightly shifting the position of a drag magnet or adjust- 
ing the dragging device, whatever it is. This adjustment up 
to a reasonable amount is provided for in meters of all types, 
and if the error is more than can be thus compensated the 
meter should be thoroughly overhauled, particularly as to the 
armature bearings. For the details of meter inspection and 
adjustments reference should be had to the instruction books 
issued by the manufacturers, as many types and forms of meters 
are in use, and no generalized directions can fit them all. 

With proper care, meters in commercial service can be kept 
correct within two or three per cent year in and year out. 
They are more apt to run slow than fast, so that the consumer 
seldom has just ground for complaint. For the best work 
meters should be installed with the idea of keeping them gen- 
erally working near their rated loads. The greatest inaccura- 
cies are at light loads, and part of the inspector's duty should 
be to make certain that the consumer's meter will start promptly 
on, say, a single 8 c.p. incandescent lamp. Otherwise the 
consumer can, and usually finds out that he can, get a certain 
amount of light without paying for it. Electric meters nearly 
always are read on their dials in exactly the manner that gas 
meters are read. With unskilled or careless men reading the 
meters there is some chance for mistake. To avert this some 
companies furnish their meter readers with record books hav- 
ing facsimiles of the meter dials plainly printed on the pages. 
The reader then merely marks on these with a sharp-pointed 
pencil the position of the hand on each dial of the consumer's 
meter, and the record thus made is translated deliberately at 
the office. Part of a page from such a record book is shown in 
Fig. 334. 

A direct-reading meter, arranged somewhat after the manner 



682 



ELECTRIC TRANSMISSION OF POWER. 



of a cyclometer, showing the total reading in plain figures, is 
a highly desirable instrument, but although several such meters 
have been brought out they have not as yet come into a secure 
place in the art. The difficulty is mainly a mechanical one. 
The meter can easily move one number disk, but, as it runs on, 
an evil time comes when it has simultaneously to move two, 
three, four, or five disks, and at one of these points it is likely 
to balk. Such a meter would be particularly hard to adapt to 
the induction type now widely used, and, desirable as it would 
be, the time of its coming is not yet. 



Customer Meter No. 

IVIeter Capacity Rate Constant 

Jan. C o qp o gr o gn ^ qr o q 

\a> s \/ V/ 5 %/\^ 9 'y \c S *y >^ ff V^ Hours. 
\^ S V' >^ s %^ \f S V^ Nf s %/\P s \/ **ours. 

Mar. n o nu ° vJC! o nCo ^ ZlZ <^ Z) 

Fig. 334, 



For special purposes a considerable variety of meters are 
used, all, however, being made and applied on substantially 
the lines already described. In some cities prepayment meters 
with an attachment for switching oh the current worked like 
a slot machine, are finding a foothold, particularly in the 
poorer quarters. Elsewhere two-rate meters with a clock- 
work attachment to cut down the rate of running between 
certain hours of relatively light station load, and some other 
automatic discount meters, have been employed. But all 
these are peculiar in their special attachments rather than in 
any fundamentals. 

The chemical meters of which the early Edison meter was a 
type, have passed quite out of use in this country. In spite 
of certain advantages, the demand for a meter which can be 



THE MEASUREMENT OF ELECTRICAL ENERGY. 683 

read by the consumer and the use of alternating current grew so 
overpowering that the chemical meter had to go. It survives 
in various forms abroad, some of them rather successful, and 
even arranged for direct reading upon an easily observed scale. 

Whatever meters and instruments are used, it is of primary 
importance that they be kept always in the best working order. 

Most of the measurements with which the supply station 
has to do are those connected with metering, but at times 
more difficult problems arise. Most of these are due to the use 
of unusually high voltage. The exact determination of high 
primary voltages is rather troublesome when one gets beyond 
the potential transformer and desires to obtain independent 
voltage measurements. The most available method of work 
is to use a high-grade voltmeter, very carefully insulated, in 
connection with very high and nearl}^ non-inductive resis- 
tances, of which the impedance has been carefully determined 
before hand. It must be remembered that such impedances 
must be added to the voltmeter impedance geometrically, as 
is generally the case in alternating current measurements. 

For a check upon such devices electrostatic instruments 
may sometimes be used to advantage. The best known of 
these is the Kelvin electrostatic balance shown in Fig. 335, 
and in its simpler forms well known in laboratories. It is 
merely a quadrant electrometer reduced to a practical form, 
and is obtainable for voltages of even 50,000 and more. It 
is not a very convenient instrument to use, but at times serves 
a useful purpose in keeping track of errors, being free from all 
those associated with the amount and phase of the current 
necessary in working electro-dynamic instruments. 

Very earnest efforts have been made to obtain a close mea- 
surement of voltage by its sparking distance between points. 
As appeared from the previous discussion of this matter, the 
measurement is a somewhat troublesome one, but it has a value 
in that it measures the very effect that is sometimes most 
important in keeping track of abnormalities of line pressure. 
From the work thus far done it appears that by careful atten- 
tion to detail, fair precision may be reached, but that it is 
unsafe to rely upon tabular values unless for the apparatus 
and conditions of use these values are checked at a few points. 



684 ELECTRIC TRANSMISSION OF POWER. 

Within limits the method is useful, and anyone interested 
in trying it will find a good account of the details in a paper 
by Fisher.* 

The measurement of line insulation on high tension systems 
is another troublesome matter. In fact, very little has been 




Fig. 335. 

done on this problem beyond the ordinary resistance mea- 
urements that may be made with the ''bridge-box," which 
should form a part of every station equipment. The capa- 
city of a long line is so considerable as to introduce great diffi- 
culties in testing with high alternating voltages, and direct 
* Trans. Int. Elec. Cong., 1904, Vol. II., p. 294. 



THE MEASUREMENT OF ELECTRICAL ENERGY. 685 

current voltages high enough to be of much use in testing are 
difficult to attain. For such measurements, for capacity 
measurements, and the like, one has to revert to strictly labo- 
ratory processes, since no commercial apparatus is up to the 
present available. 

No useful general method of locating faults on high tension 
overhead lines has yet been devised. They occur under so 
various conditions and for so different causes that they cannot 
be treated in any systematic way. The telephone and friends 
along the line is the winning combination in case of trouble. 
As a rule line troubles are not instantaneous in their occurence, 
and serious results can often be averted by starting a prompt 
inquiry over the wires. On high voltage systems any abnor- 
mal loss of energy means mischief in the very near future, so 
that there is the constant necessity of keeping the insulation 
at the highest attainable figure. If it is low enough to measure 
readily, it is too low for safety. On underground circuits 
which are usually of rather moderate voltage and length, 
troubles assume a more definite character and can generally 
be located when the cable can be put out of service and tested. 




Insulation tests are here of more service and should be periodi- 
cally made. In case of grounds the following adaptation of 
the loop test, described by Ferguson in a valuable paper on 
underground work,* may be found useful. Fig. 336 shows 
the arrangement of the apparatus. AB is sl slide wire bridge 
or its equivalent, carefully calibrated. C is the moving con- 
tact connected to a grounded battery. Let a conductor be 
grounded at G. Join the remote end of it to the end of a 

* Trans. Int. Elec. Cong., 1904, Vol. II., p. 683. 



686 ELECTRIC TRANSMISSION OF POWER. 

sound conductor and the near ends respectively to the ends 
of the bridge. Then balance in the ordinary way. Let L 
be the total length of the joined conductors. Then 

^^- CB • 

L may be taken directly as linear distance if the loop is of 
the same cross section throughout, but if the sizes differ L 
should be taken as resistance and A G should be reduced to 
distance from the known size of the conductors. The result 
does not involve the resistance of the fault, but this should 
be low enough, or the testing voltage high enough, to get proper 
deflection of the galvanometer. This test is reported to give 
location within one or two hundred feet, in lines from 1 to 
5 miles in length. If the conductor is burned off, the fault 
can sometimes be located by capacity tests from the two 
ends, if there is not too much leakage. Similar tests can in a 
certain number of cases be used for overhead lines, for which 
reference should be had to the general testing methods used 
on telegraph systems, but as before noted a fault on a high 
tension line is generally of so pyrotechnic a character that it 
can be located closely enough for the repair gang long before 
the line can be cleared and the necessary measurements made. 



CHAPTER XVIIL 

PRESENT TENDENCIES IN HIGH VOLTAGE TRANSMISSION. 

It is now nearly seven years since the third edition of this 
work appeared, and during that time there has been a great 
advance in the freedom with which high voltage is employed, 
although there have been no sensational changes. Improvement 
has come through gradual progress along lines which had already 
been pretty well mapped out. 

In fact, the list of high voltages in existing plants runs little 
higher to-day than it did five years ago, albeit the average work- 
ing voltage, if one may be permitted to speak of so vague a 
thing, has been nearly doubled within the same period. The 
list of high voltage transmissions which appears at the end of 
this chapter, tells the story clearly enough. It has proved so 
hopeless a task even to catalogue the 10,000 volt plants that 
it has been necessary to confine the list to those plants operated 
at 20,000 volts or more. There are about 110 such plants in the 
United States, Canada, and Mexico, as against 70 plants working 
at or above 10,000 volts five years ago. And of the total list at 
least 25 are working at or above 50,000 volts, in contrast with 
the single plant of the earlier date. 

The longest distance of transmission in the earlier list is 145 
miles, on the same great system which has now carried commer- 
cial transmission up to 232 miles. The region between 40,000 
and 60,000 volts has now been pretty thoroughly explored, and 
may be entered without fear. The difficulties encountered there 
are, as was to be expected, connected with the line insulation. 
So far as transformers are concerned, higher voltages than 
60|000, perhaps up to 80,000 or even 100,000, might be commer- 
cially employed, but the insulator has not kept pace with the 
transformer, and while excellent insulators have been made for 
use, at 60,000 volts and even a little higher, the factors of safety, 
generally 2.5 to 3, are not yet as great as conservative engineer- 

687. 



688 ELECTRIC TRANSMISSION OF POWER. 

ing should demand. That this condition will be improved there 
is little reason to doubt, but for the present great caution is 
desirable in going above 60,000 volts. 

The most radical innovation in high voltage construction is the 
introduction of the tower construction with spans of 500 ft. or 
more. The tower system gives certainly a very durable line, 
and one which, under some conditions, is relatively cheap. Its 
weak point is exceptional danger from lightning, and the certainty 
that any insulator failure will put the line out of business unless 
current is immediately shut off on any sign of a ground as is the 
custom on some tower systems. The use of thoroughly treated 
wooden cross arms would probably relieve this difficulty to a 
considerable extent. 

As to distance, the question is now as it always has been, a 
commercial one. The higher the available voltage, at least 
within wide limits, the greater distance can be covered with a 
given capital and maintenance charge per kilowatt transmitted. 
Certain elements of cost like right of way, poles, insulators, and 
line construction depend mainly upon the distance alone and not 
upon the output, so that in a general way the larger the amount 
of power to be transmitted the farther it will pay to transmit it 
irrespective of voltage, which in every case of long transmission 
is likely to be pushed up as far as the state of the art permits. 
At the present time power is regularly transmitted 100 miles or 
more from about a dozen plants, but the ordinary requirements 
are, and are likely to remain, very much below this figure. 

As a matter of fact, there are comparatively few sources of 
power which are compelled to find a market at a great distance 
or are large enough to warrant a very long transmission. In 
most cases the power can be sold within a radius of much less 
than 100 miles. Still, there are instances in which conditions 
demand a far greater distance of distribution. At the present 
time enough experience has actually accumulated to justify 
transmissions of several hundred miles, so far as the engineer- 
ing side of the matter is concerned. 

Few data on the economic performance and cost of mainte^ 
nance of very long lines are available. The latter item un- 
doubtedly increases considerably faster than the length of 
the line since the actual number of troubles increases, other 
things being equal, about as the number of insulators, while 



HIGH VOLTAGE TRANSMISSION. 689 

they are scattered over a large territory that must be watched. 
This fact has a bearing on the advantage gained Y\^here very 
large amounts of energy are transmitted. 

In the matter of commercial frequency there is small ten- 
dency toward change. A large majority of all the high tension 
plants, including nearly all of those operating over 100 miles or 
more, are worked at 60^. One of the remaining ones is worked 
at 50^, the others at 25^. In the case of a transmission of 
several hundred miles involving say 50,000 or 100,000 KW, a 
lower frequenc}^ than 60^would certainly be advisable, but 
for the rank and file of plants there is a tendency to standard- 
ize at GO^unless there is some very gocd reason to the con- 
trary. 

As to generator voltage, practice has not been much changed 
recently. With the increasing use of 20,000 volts and upwards 
there is perhaps somewhat less incentive to use high voltage 
generators, which now show an economy only on lines of a few 
miles in length. Nevertheless, many generators of 10,000, 
12,000, and 13,500 volts are in use, the first mentioned having 
been superseded in new plants by the others. For use with 
raising transformers of course any voltage will serve, but prac- 
tice is now gravitating toward about 2,200 to 2,400 volts which 
is standard for local lighting and power distribution. 

Occasionally a somewhat higher voltage is chosen on account 
of a more extensive local load than can be conveniently man- 
aged at 2,200 volts, but such instances are exceptional. 

The most striking and important feature in recent power 
transmission work is the growing tendency to unite the power 
generating plants of a single district into a coherent system. 
This means far more than the fusion of the business of several 
stations into a single administration — it implies as well the 
physical organization of a group of plants into a single dynami- 
cal unit. It must not be confused with the tendency to replace 
a group of stations by a common central plant, a practice often 
carried to unwise extremes. 

The development of the transmission network is carrying 
out upon a gigantic scale the same organization that has proved 
so valuable in low tension distribution networks. It consists 
in linking together into a network the transmission lines of all 



690 ELECTRIC TRANSMISSION OF POWER. 

the power plants of a large region, so that each may reinforce 
the others in capacity and in the market for output. The 
region covered may amount to thousands of square miles, and 
the stations linked may be half a dozen or more, scores of miles 
apart and located on different streams, and even upon differ- 
ent watersheds. It has proved feasible to operate many plants 
in parallel on such a network whether large or small, driven by 
water- or by steam-power. 

The essential feature is that the network voltage shall be high 
enough to enable the plants to work together without a loss 
in the lines sufficient to imperil regulation. 

If the network be wisely laid out it will like low tension net- 




works, enable the territory to be covered at a lower cost for 
lines than if independent feeding systems were employed, or 
for equal costs it will give a lower average loss of energy. 

There is also a considerable gain in the matter of establish- 
ing reserve capacity, since in case of accident the several plants 
can help each other out. In the same way, with a properly 
arranged network one line so serves as a relief for another as 
to obviate the necessity for duplicating lines. 

The details of networks are very various and there are no pre- 
cise rules to be laid down, but the general principles are shown 
in Figs. 337 and 338. The former shows the ordinary arrange- 



HIGH VOLTAGE TRANSMISSION. 



691 



ment of independent stations, the latter the effects of intelli- 
gent linkage. In Fig. 337 there are four generating stations 
1, 2, 3, 4, and three load points A, B, C. We will assume 1 
and 4 to be the most important stations, and B the largest load 
point. Now as power plants are commonly installed by di- 
verse interests and somewhat at haphazard^ one would gen- 
erally find say three companies working, one supplying A 
from station 4, another supplying B and C from station 1, and 
a third supplying B from stations 2 and 3. The hues cer- 
tainly, and the pole lines generally, would be in duplicate, and 
the voltages would differ according to the period of the re- 
spective installations. In point of fact the largest network 




Fig. 338. 

in existence is the result of a consolidation which left the oper- 
ating company in the proud possession of lines at 50,000, 
40,000, 23,000, 16,000, 10,000, and 5,000 volts, and it is small 
wonder that the work of standardization has been long de- 
layed. 

Now, were the situation of Fig. 337 developed in accordance 
with the methods now becoming current, the result would be 
something like Fig. 338 modified more or less by topographical 
and commercial requirements. Here there are no duplicate 
lines as such, but each load point is supplied by two or more 
lines through each of which all the generating stations can 
deliver current. 



692 ELECTRIC TRANSMISSION OF POWER. 

The security against interruption is further increased by 
the fact that the several supply lines to each load point follow 
different routes. As regards auxiliary plants and spare capa- 
city the network of Fig. 338 is highly advantageous, since a 
single auxiliary plant, say at B, will serve for the whole system, 
and the reserve generator capacity can be located wherever 
it seems best. 

The price paid for these advantages is some additional loss 
in the lines at times when the longer routes are in action, and 
some additional care in operation. 

The latter requirement comes not so much from any one 
cause as from a variety of causes. As a general proposition, 
a group of stations can be operated in parallel without much 
difficulty provided the stations individually are well designed. 
The first requirement is stability of voltage at the several 
stations, which implies in turn generators giving close regula- 
tion, especially under changes of lag, and operated at constant 
speed. Second, there should not be excessive drop in the 
lines, for changes of terminal voltage due to this cause make 
it difficult to equalize the loads between the stations. Third, 
there should be means for governing the power factors so as 
to steady the inductive drop and to keep down cross currents 
between the stations. 

The best modern practice tends toward throwing the bur- 
den of regulation upon the sub-station, the endeavor of the 
power houses being to preserve uniform voltage at the ends 
of the respective lines. The details of regulation are then 
looked out for by the sub-Suation regulating apparatus, voltage 
regulators, or synchronous motors at adjustable excitation. In 
this operation the power factor meter plays an important 
part. 

Obviously a scheme of regulation such as this cannot be 
carried out effectively unless it is accomplished at a single 
point and under systematic direction. The regulating point 
is naturally the main substation as at B, Fig. 338. For such 
a single point the regulation can be reduced to a rather regular 
programme, but the wandering of the load which takes place 
on every large system complicates the situation. For ex- 
ample, at certain times of the day, A, Fig. 338, may require 



HIGH VOLTAGE TRANSMISSION. 693 

an abnormal proportion of the total load, and the voltage 
regulators must be adjusted accordingly; the exact programme 
being determined by experience. The problem is akin to 
voltage regulation upon a large distributing system, and must 
be solved by the same general process. 

As in distributing networks, too, means must be provided 
for promptly isolating lines on which there is trouble, and to 
this end it is often necessary to do a certain amount of switching 
at high tension. If, for example, the line B 3, Fig. 338, begins 
to show signs of trouble, quick work in cutting it clear may 
often save a short circuit that would seriously disturb the 
voltage of the whole system. 

When a group of stations of very diverse character is to be 
operated in parallel great care must be taken with the gov- 
erning. If a sudden variation of load occurs, the natural 
tendency is for the shock to be taken up by the plant equipped 
with the most sensitive governor. This would practically 
mean that if a steam plant were one of the group it would 
have to stand the worst of the blow, which would then fall in 
succession upon the hydraulic plants in order of the rapidity 
of their governing. As it is undesirable generally to make 
the steam plant take up such variations, the governers in the 
several plants should be adjusted for rapidity of action in 
such a manner if possible, as to throw the shock on the plant 
best able to stand it. 

The large networks of the country have grown up rather 
gradually so that they have not been arranged as yet to oper- 
ate in the fullest harmony, but they are being steadily im- 
proved. 

The most striking single example of a great and far-reaching 
system is that of the California Gas and Electric Co., of which 
much has been heard. It is shown roughly in Fig. 339, which 
gives not only the system but the parts from which it has 
been aggregated. It operates about 700 miles of line at 
50,000 volts, besides several hundred miles at lower voltages, 
and has an aggregate capacity of about 50,000 KW. There 
are all told 14 power houses, the most recent being a huge 
gas-engine auxiliary plant in San Francisco, with 4,000 KW 
units, the first of which has just been installed. As will be 



694 



ELECTRIC TRANSMISSION OF POWER. 



seen from the map, the fusion of the whole into a network is 
not yet complete, but it is being done as occasion offers. The 
transmission from the De Sabla power house to Sausalito, 



L A S S E R 



- 




_ Honey ijm. ■ 1 


p 


L U 


MAS ) 1 




M MateoVN:^^ \ Mission Sau '^osp- ST A N~IS LA U S 

>^MATE(^>:^»^.= ^-— \ \ ^■ 

dvvood^/Cr;^;r^,v\ arm Springs V,^ \^- 



MERCED 



1 aio Alto 



lAlviso 
"lg) San Jose 
SANTA CLARA 



\ 



Fig. 339. 



232 miles, is the longest yet attempted in the world although, 
on occasion, power has been commercially transmitted be- 
tween points on the system nearly 350 miles apart. The 
main transmission may be reckoned at about 150 miles from 



HIGH VOLTAGE TRANSMISSION. 695 

the chief power houses at Colgate and Electra. On this sys- 
tem have been worked out some very important problems 
in power transmission. The long cable span over Carquinez 
Straits has already been described, and it need only be added 
that during more than three years of operation it has given 
no trouble. Long spans are freely used on the more recent 
parts of the system with a strong wooden pole construction. 
Another interesting feature of the system is the considerable 
use of long break open-air disconnecting switches on lines 
up to more than 60,000 volts, and also the practical abandon- 
ment of ordinary lightning arresters in favor of open-air horn- 
gap arresters of the simplest possible description. The whole 
system spans a space of about 240 miles in length by about 
half that breadth, and constitutes altogether the most exten- 
sive power-transmission yet undertaken, supplying light and 
power at nearly a hundred distribution points. The uniform 
frequency is 60^ and the voltage is tending toward 60,000 as 
the general limit for the present. 

Second only in magnitude to this system is that of the 
Los Angeles Edison Co., in southern California. This was 
earlier than the northern system in its inception, containing 
among its constitutents not only the first polyphase trans- 
mission plant operated in America, but the first long distance 
plant operated at anything like the voltages now common. 
It is less characteristically a network than the system just 
described, being essentially a long trunk line through the 
splendid valley that lies south of the Sierra Madre, beginning 
in the mountains just east of Redlands and running clear 
through to the sea, with numerous branches, the main point 
of supply being Los Angeles itself. The system operates 8 
plants, five hydraulic and three steam, with several more hy- 
draulic plants under construction. Fig. 340 shows in outline the 
group of plants and transmission lines at present constituting 
the system. The beginning of the network was the Redlands 
plant, known on the map as Mill Co. Hyd. P.H. 1, the first 
polyphase transmission plant, started as a 2,500 volt trans- 
mission into Redlands in 1893. Three years later the Edison 
Company started with a steam station in Los Angeles, and 
in 1898 it acquired the Southern California Power Com- 



696 



ELECTRIC TRANSMISSION OF POWER. 



tfilf-u,/ 




HIGH VOLTAGE TRANSMISSION. 697 

pany, which then owned the original Redlands plant and the 
Santa Ana canon plant with its 80 mile transmission at 33,000 
volts into Los Angeles, the first of the very long high voltage 
lines. Since then its growth has been rapid. In 1896 the 
Mill Creek plant was changed by extending the pipe line, from 
377 to 530 ft. head, and raising transformers to 10,000 volts 
were installed, although it is interesting to note that the origi- 
nal generators are still in use after more than 12 years of ser- 
vice. Later, Mill Creek power houses No. 2 and No. 3 were 
added higher up the caiion. Also in Santa Ana cafion a 
plant, No. 2, has been added, and No. 3 and No. 4 are 
projected. In the same region the Lytle Creek Power House 
has been added on a branch of the Santa Ana River. A 
point worth noting in several;4;)f these later plants, is that 
the receiver, a usual feature of the earlier hydraulic plants, 
and already mentioned, has been abandoned in favor of 
branches spreading finger-like from the end of the main 
pressure pipe, cast-steel Y's being used for the division. 
This arrangement averts some loss of pressure otherwise 
incurred. 

Another feature lately introduced in the hydraulic construc- 
tion, is the use of concrete pipe, on the slight grades leading to 
the steel pressure pipes. This pipe is moulded on the ground 
of heavy gravel 2 parts, and Portland cement 1 part, made up 
in very short sections and united by concrete collars. It is 
laid in trenches and back filled. Depressions across which this 
pipe cannot be conveniently laid are spanned by steel pipe in 
inverted siphons. Aside from the hydraulic plants here noted, 
the system is to be supplied with a very large additional amount 
of power from points on the. Kern River far to the northward, 
over a transmission system of nearly 150 miles in length. The 
number of power sites available is 6, aggregating some 87,000 
HP at the minimum, but of these only plant No, 1 is needed for 
immediate use, and that with some 24,000 HP capacity is near- 
ing completion. A notable fact is that the whole Kern River 
district is across the Sierra Madre Mountains, on a watershed 
of its own covering some 2,000 square miles, and has a higher 
and more wooded region upon which to draw than is possessed 
by the streams earlier developed. ,: 



698 



ELECTRIC TRANSMISSION OF POWER. 



The whole system is operated at 50^, which was the fre- 
quency adopted for the original Redlands plant. 

As in the work of the California Gas and Electric Company, 
there is here also a tendency to use much longer spans than are 
common in regions where transmission lines are less familiar. 
Fig. 341 shows the pole head used for some miles of recently 
constructed line. Of course, anyone who takes the trouble to 
design a pole line instead of guessing at it knows that a 225 ft. 




iiG. 341. 



span such as is here used is entirely feasible with light high 
voltage wires, but it is a rather striking exhibit to see the plan 
carried out with a 12 ft. upper cross arm. The line thus con- 
structed stood up against wind storms of exceptional violence 
without the slightest damage. It is generally preferable to set 
the wire triangles with the points up instead of down as here 
shown, and in most cases so long a cross arm as here shown 
would scarcely be needed. 



HIGH VOLTAGE TRANSMISSION. 699 

The important group of plants near Salt Lake City, Utah, 
has already been noticed. The system just over the moun- 
tains near Telluride, Colorado, although much less extensive, is 
memorable as the scene of the first serious work on transmission 
at high voltage from which was derived much of the data and 
experience which has made modern transmission practicable. 

The transmission network fed from many stations must 
certainly be ranked as the most considerable advance in power 
transmission made within recent years. As at present carried 
out it is mostly concerned with large powers, but the same 
principle applies whatever the scale of the operations. 

There are many small powers which can well be utilized in 
a similar manner. The tendency is to work up the large 
powers and to neglect the lesser ones. 

Another line of operations which is beginning to be pressed 
is the creation of artificial powers. In regions of fairly con- 
siderable rainfall the aggregate amount of water received by 
a given watershed may be large, while the distribution of run- 
off is very unfavorable. If the situation is such that even a 
few square miles of watershed can be made to contribute to 
a storage system at high head, a very considerable permanent 
power can be developed. 

The problem is akin to the ordinary one of providing water 
supply for city use or for irrigation, with the exception that 
for a power plant the available head should be as great as 
possible. 

Oddly enough the most typical case of the kind here con- 
sidered is to be found in the State of Vermont. In the south- 
ern reaches of the Green Mountains there is much high country 
over which the rainfall is heavy, averaging nearly 45 inches per 
annum, and at a point in this region about eleven miles from 
Rutland on East Creek, a water storage for power purposes has 
been created. The area of the pond is about 800 acres, secured 
by a dam 750 ft. long and 54 ft. in maximum height. The 
storage here secured amounts to 435,000,000 cubic feet and 
by a pipe line 35,000 ft. long the water can be delivered under 
a head of 697 ft. At the present time the water from the large 
reservoir is used to reinforce the supply in a second reservoir 
5 miles down stream having a capacity of about 63,000,000 



700 ELECTRIC TRANSMISSION OF POWER. 

cubic feet, and this lower reservoir supplies the power station 
under a head of 222 ft., through a pipe hne 8.000 ft. long. Thus 
at present only about the lower third of the fall is utilized, 
though the rest can readily be made available. 

The drainage area of the upper reservoir is only 15 square 
miles, but of a total rainfall of 45 inches only 11.5 inches need 
reach the pond to fill it, while the actual run-off from the water- 
shed has been found by measurement to be nearly 70 per cent, 
so that there is a large margin of safety in reckoning the stor- 
age given. 

At the full head given the 435,000,000 cubic feet of storage 
would give more than 5,000,000 KWH per year, available as 
desired, and equivalent under ordinary conditions of use to 
an installation of more than 2,000 KW. Even now 1,200 KW 
of generator capacity is in place, and the maximum output 
provided for is merely a question of profitable use. 

The cost of purely artificial storage is generally high and 
only in case of very great heads is it likely to pay at present. 
If topographical conditions are favorable, however, there is 
no reason why impounding the rainfall cannot be made profit- 
able. The actual amount of land diverted from its ordinary 
uses is in such a case only that used for the reservoir, in which 
this power storage has the advantage of storage for water 
supply, and stands in exactly the position of storage for irriga- 
tion. In the Chittenden reservoir only some 800 acres of 
upland was removed from employment as farm, forest, or pas- 
ture. As regards the rest of the watershed, it is rather improved 
than injured by the pond. 

Looking at the proposition broadly, one can under a head 
of 650 to 700 ft. store power on the basis of 1 mechanical horse- 
power hour from the wheels for each 60 cubic feet of water, or 
about 4,800 KWH for each acre per foot of depth. And for 
each acre of watershed one should be able at ordinary values 
of the run-off, to store this foot of depth. Mountain land, 
therefore, may easily be worth more for storage than for any- 
thing else. At present, water-powers can generally be de- 
veloped from natural falls more easily than they can be thus 
created, but as the natural powers are taken up and fuel rises 
in price storage will become more and more profitable. 



HIGH VOLTAGE TRANSMISSION. 701 

In the inevitable struggle of industry against increasing 
scarcity of fuel, every source of hydraulic power must be de- 
veloped to the fullest extent. It is idle to speculate on the 
date of probable exhaustion of the coal supply since we know 
not what stores are hoarded unknown in the great unused con- 
tinents of Africa and South America, but the fact remains 
that to-day the energies of the human race are being expended 
in regions in which fuel is necessary not only for industry but 
for artificial heat, and that the supply readily obtainable is 
being rapidly depleted. 

At no distant day increasing cost of fuel will compel either 
a drift of civilization southward or the utilization of every 
continuous source of energy available. The key to the situa- 
tion lies in the transmission of power at high voltage and in the 
union of all the available powers of a large district into a coher- 
ent network. The steady rise in working voltage during recent 
years makes possible an ever increasing area over which net- 
works can be made effective. 

The following approximate list of plants now operating at 
20,000 volts and more, tells more plainly than any general state- 
ment the tendencies now prominent. But steps are now going 
forward toward still higher voltages, 70,000 to 80,000 being 
already in view, and for some of the transmissions now being 
seriously considered, like that from Victoria Falls on the Zam- 
besi to the Rand, something like double even these figures is 
imperative for commercially profitable work. 

The highest voltage now regularly in commercial use is 69,000. 
Then follows a group of 60,000 volt plants some 15 in number, 
and while some of these are working slightly under the rated 
pressure most of them are fairly up to the limit. On the whole 
these high voltage systems are being worked with little if any 
more trouble than those at half the pressure, and at the present 
moment 60,000 volts may be regarded, as in a measure, standard 
pressure for very long transmissions. Above this experience is 
still very limited, and while some large transformers have already 
been shipped insulated for voltage as high as 80,000, they are, so 
far as yet announced, actually worked in star connection or on 
taps for lower voltage. The Los Angeles Edison Co. which has 



702 ELECTRIC TRANSMISSION OF POWER. 

done much pioneering in high pressure work, is experimenting 
with 75,000 volts on its Kern River line, and it is very likely 
that this plant and others will soon regularly pass the 70,000 
volt mark to greater or less extent. 

The insulator is the only thing which stands in the way. 
Very recently material progress has been made in insulators 
carrying the wires in suspension, and thereby avoiding the very 
serious difficulty of supporting the structure necessary to give 
proper sparking distance for voltages of 100,000 volts or more. 
The experimental results from such insulators are good, but they 
have not yet been tried out to any such extent as is necessary 
thoroughly to test their mechanical properties. At the present 
high prices of copper and aluminum the need of a general 
increase of voltage is pressing. In case of very long lines the 
cost is very severe even at the highest voltages now in use. If 
the suspension insulators prove effective and reliable, there will 
be a good change for considerable increase and corresponding 
economy. 

The present list shows how fearlessly voltages considered 
extreme a few years since, are now employed under all sorts of 
climatic conditions, and this fact is the best possible augury for 
greater achievements in the future. 



ELECTRIC TRANSMISSION OF POWER. 

American Plants Worked at 20,000 Volts or More. 



703 



Name. 


Location. 


Capa- 
city, 
KW. 


Vol- 
tage. 


■4 


4 


Grand Rapids-Muskegon Power 

Co. 
California Gas & Electric Co 

American River Electric Co 

Ontario Power Co 

Winnipeg General Power Co 

Electrical Development Co. of 

Ontario. 
Canadian Niagara Power Co . ... 

Washington Water PoAver Co. . . 
Guanajuato Electric & Power Co. 
Whitney Reduction Co 


Roger's Dam, Mich.. 

Colgate, Electra, De 
Sabla, and else- 
where, Cal. 

Placerville, Cal 

Niagara Falls, Can. 

Lac Du Bonnet, Man- 
itoba 

Niagara Falls, Ont.. 

Niagara Falls, Ont. . 

Spokane, Wash 

Guanajuato, Mexico. 

Salisbury, N. C 

Wisconsin 

Kern River, Cal 

Duluth, Minn 

Helena, Mont 

Lac Du Bonnet, 

Winn. 
Canyon Ferry, Mont. 
Seattle, Wash 

Bishop's Creek, Cal. 
Taylor's Falls, Mich. 

Montreal, P. Q 

(Shawinigan Falls). 

Anderson, Ind 

Durango, Colo 

Taylor's Falls, Minn. 

Hamilton, Ont. 

(Decew Falls) 

Lewiston, Id 

Clarkston, Wash... 

Asotin, Wash 

Warrior's Ridge, Pa. 
Great Falls, S. C... 

Provo, Utah 

Norris, Montana 

Kalamazoo, Mich.. . . 

Kalamazoo, Mich 

Plainwell, Mich 

Logan, Utah 

Near New Milford, 

Conn. 
American Fall8,Idaho 

Bull's Bridge, Conn. 

Auburn, N.Y 

Cedar Rapids, la.... 

Sacramento, Cal 

Ft. Wayne, Ind 

Honolulu, T. H 

Indianapolis, Ind... 

Lebanon, Ind 

Los Angeles, Cal 

Snoqualmie, Wash.. 
Lima, Ohio 


3,000 
50,000 

3,000 

30,000 

5,000 

88,000 

37,500 

9,000 

2,400 
25,000 

2,000 
20,000 
22,500 

4,500 
14,000 

7,500 
23,000 

1,500 
10,000 
15,000 

3,000 
4,500 
10,000 

{ 2,200 

j 900 

2,000 
24,000 
1,500 
2,000 
1,500 
600 
2,500 
2,000 
6,000 

500 

4,500 
1,300 
1,160 

590 
6,900 

1,350 
3,000 
2,000 
750 
11,000 
1,900 
3,000 
3,000 
4,500 
3,000 
1,875 

1,350 
14,000 


69,000 

15,000 
60,000 

60,000 
60,000 
60,000 

60,000 

22,000 
40,000 
60,000 
60,000 
60,000 
60,000 
60,000 
60,000 
60,000 
60,000 
60,000 

57,000 
(55,000 
140,000 
50,000 
50,000 
50,000 

50,000 
50,000 
50,000 

{ 45,000 

j 45,000 

45,000 
44,000 
40,000 
40,000 
40,000 
40,000 
40,000 
40,000 
33,500 

33,500 

33,500 
33,000 
33,000 

33,000 
33,000 

33,000 
33,000 
33,000 
33,000 
33,000 
33.000 
33,000 
33,000 
33,000 
33,000 
32,000 

30,000 
30,000 


58 

(139 

145 

(232 

90 

"67 

160 

|. 

110 
101 
75 
60 
139 

'"60 
65 

65 

113 
38 
80 

"55 
41 

30 

"55 
67 
46 
32 
75 
150 
30 

25 

40 

"46 

"28 
80 
80 
80 
37 

36 
35 


30 

1" 

60 
25 
60 

25 

25 

60 
60 
50 


ATnHienn Rivfr Power Co. 


60 




50 


Gi'eat Northern Power Co 


25 
60 


Winnipeg Electric Railway Co. . 


60 
60 


Columbia Improvement Co 

Nevada Power & Milling Co 

Columbia Improvement Co 

Shawinigan Water & Power Co. . 

Union Traction Co. of Indiana.. 

Animas Canal Co 

Minneapolis General Elec. Co... . 
Hamilton Cataract Power Light 
& Traction Co. 


60 

60 
60 
30 

27 
60 
60 

J66 

u 


Juniata Hydro-Electric Co 

Southern Power Co 


I 

60 
60 


Telluride Power Transfer Co 

Power Co. of Montana, The 

Kalamazoo A'alley Electric Co.. . 

Detroit and Chicago Ry. Co 

Plainwell Construction Co 

Hercules Power Co.. 


60 
60 
60 
60 
60 
60 


New Milford Power Co 


60 


American Falls Power Light & 

Water Co. 

Conn. Railway & Light Co 

Auburn and Syracuse El.R. R. Co. 
Cedar Rapids Electric Light & 

Power Co. 
Central California Electric Co.. . 
Fort Wayne & Wabash Valley 

Tr.Co. 


60 

60 
25 
60 

60 
25 

60 


Indianapolis & Cincinnati Tr.Co. 
Indianapolis & Northern Tr. Co.. 

San Gabriel Electric Co 

Seattle-Tacoma Power Co 

Western Ohio Ry. Co 


25 
25 
60 
60 
26 


Spring River Power Co. 


Joplin, Mo 

Redlands, Cal 

Redlands, Cal 

Crafton, Cal 


25 


Southern Cal. Power Co 

Redlands Elec.Light & Power Co. 
Edison Electric Co 


50 
50 
50 


Economy Light & Power Co., 
Joliet, 111. 

Mt. Whitney Power Co 

Hudson River Electric Co 


Lockport, 111 

Visalia, Cal 


60 
60 


Sjpiers Falls, N.Y... 


40 



704 



HIGH VOLTAGE TRANSMISSION. 



American Plants Worked at 20,000 Volts or 


More — 


■ Continued. 




Name. 


Location. 


Capa- 
city, 
KW. 


Vol 
tage. 


■ « 

■^9 


it 


Montgomery Water Power Co... . 

North Mt. Power Co 

Aurora, Elgin & Chicago Ry 

Columbus, London & Springfield 
Ky. 

Montana Power Trans. Co 

Columbia Mills Co 


Tallassee,' Ala 

Junction City, CaL.. 

Wheaton, 111 

Medway, O 

Butte, Mont 

Columbia, S. C 

Ogden, Utah 


2,625 
1,500 
4,500 
1,600 

3,000 
1,000 
3,750 
1,350 
2,800 
11,000 

6,000 

3,000 
10,000 
2,250 

6,000 

8,750 

3,100 

1,100 

2,000 

16,000 

10,500 

4,000 

4,500 

500 

78,750 

1,500 

900 

1,000 

1,200 

2,250 

37,500 

2,250 
5,370 

750 
2,000 

600 
1,750 

750 
1,500 
4,000 
2,500 

500 

1,500 
5,250 
1,200 
1,000 
15,000 
1,125 

1,500 
4,500 

6,000 
600 

2,650 

1,410 

600 


30,000 
30,000 
26,400 
26,400 

26,000 
25,000 
25,000 
25,000 
25,000 
25,000 

25,000 

25,000 
25,000 
23,000 

23,000 
23,000 
22,500 
22,500 
22,500 
22,000 
22,000 
22,000 
22,000 
22,000 
22,000 
22,000 
22,000 
22,000 
22,000 
22,000 

22,000 

22,000 
22,000 

22,000 
22,000 

22,000 
22,000 

22,000 
22,000 
22,000 
22,000 

22,000 

22,000 
22,000 
22,000 
22,000 
22,000 
22,000 

22,000 
22,000 

22,000 
22,000 

20,000 
20,000 
20,000 


30 
65 
28 
59 

21 
18 
40 
40 
17 
19 

45 
31 

28 
25 
90 

12 

"24 
33 

"n 

12 

"32 
22 
33 
10 

27 

"ie 

25 

28 
35 

100 

22 

12 

40 
60 

"22 

38 
17 

30 


60 
25 
25 
25 

60 
40 
60 


Blue Lakes Water Co 

Montreal Cotton Co 


Big Bar Bridge, Cal. 

Vallevfield, Que 

Chambly Falls, St. 
Therese 

Snoqualmie Falls, 
Wash. 

St. Paul, Minn 

Kakabeka Falls,Ont. 

Salt Lake City, (Bear 
River) U. 

Trenton Falls, N. Y. 

York Haven, Pa 

Belton, S. C 

Phoenix, Ariz 

Hamilton, Ont 

Pittsburg, Pa 

Morgan Falls, Ga. . . . 

Utica, N. Y. 

Tlalnepantla, Mex.. 

Yreka, Cal 

Niagara Falls, N. Y. 

Truckee River, Cal.. 

Boise, Id 

Boise, Id 

San Jose, Cal 

Cascade, B. C. (Co- 
lumbia River) 

Niagara Falls, N. Y. 

Pachuca, Mex 

Mexico, Mex 

Tolo, Ore 

Elyria, Ohio 

Orilla, Can. 


60 
60 


Montreal Light, Heat & Power Co. 

Snoqualmie Falls Power Co 

St. Croix Power Co 


60 
60 
60 


Kaministequia Power Co 




Utah Sugar Co 

Utica Gas & Electric Co.. 


60 
60 


York Haven Water Power Co. . . . 
Belton Power Co. 


60 
60 


Phoenix Lighting & Fuel Co 

Cataract Power Co 

Allegheny County Light Co 

Atlanta Water & Elec. Power Co. 
Utioa Electric Light & Power Co. 

G. & 0. BranifE & Co. 

Siskiyou Electric Power Co 

Niagara Falls Power Co 

Truckee River Power Co 


60 
66 
60 
25 
60 

'eo' 

25 
60 
60 


Boise-Payette Electric Power Co. 

Big Creek Power Co 

Cascade Water Power & Lighting 
Co. 

Cataract Power & Conduit Co., 
(Niagara Falls Power Co.) 

Cia. Electrical Irragadora 

Cia. Explotadora de las Fuerzas 
Hidro-Electricas de San Ilde- 
fonso. 

Condor Water & Power Co 

Cleveland & Southwestern Trac- 
tion Co. 

Corporation of Or ilia 

Detroit, Ypsilanti & Ann Arbor 
R. R. Co. 

Grande Consolidated N.S.&P.Co. 

Highland Park Mfg. Co 


60 
60 
60 

25 

60 

60 
25 

60 


Ypsilanti, Mich 

Grand Forks, B.C.. 

Charlotte, N. C 

S.Chicago, 111 

Scranton, Pa 

St. Catherine, Can... 

Myrtle Falls, Wash. 

Redding, Cal 

Portsmouth, Ohio. . . 

Alliance, Ohio 

Massena Springs,N. Y. 
Silver City, Id 

Floriston, Cal 

Vancouver, B. C. 

(Lake Beautiful). 
Connellsville, Pa.... 
Youngstown, Ohio . 

West Kootnay, B. C. 
Colo. Springs, Colo.. 
Mercur, Utah 


25 

60 
60 


Illinois Steel Co 


25 


Lackawanna & Wyoming Valley 

Rapid Trans. Co. 
Niagara, St. Catherine & Toronto 

Ry. Co. 

Nooksack Falls Power Co 

Northern California Power Co.. . 
Portsmouth St. Ry. Co 


25 

25 

60 
60 
60 


Stark Electric Ry. Co 

St. Lawrence River Power Co — 
Trade Dollar Consolidated Min- 
ing Co. 

Truckee River Wr. Power Co 

Vancouver Power Co. 


25 
25 
60 

60 
60 


W. Penn. Ry. & Ltg. Syndicate. . 
Youngstown & Sharon Railroad 

& Lighting Co. 
West Kootnay Electric Power Co. 

Colorado Electric Power Co 

Consolidated Mercur Gold Mines 

Co. 


60&25 

60 

60 
30 
60 



ELECTRIC TRANSMISSION OF POWER. 



705 



American Plants Worked at 20,000 Volts or More. - 


- Continued. 




Name. 


Location. 


Capa- 


Vol- 
tage. 






G. & 0. Braniff & Co 


Deronica, Mex 

Jenison, Mich 

Anderson, Cal 

Lowville, N. Y 

Quebec, P. Q 

Canada 

Colton, N. Y 


i',666 

1,500 
250 
1,500 
1,5C0 
1,500 


20,000 
20,000 

20,000 
20,000 
20,000 
20,000 
20,000 


"is 
"'so 


50 


Grand Rapids, Holland & Lake 
Mich. Railway Co. 

Keswick Electric Power Co 

Welmore Electric Co 

Jacques Cartier Power Co 

International Hydraulic Co 


25 

60 
60 
60 
60 
60 









This list cannot claim to be complete, but it is approximately so at the date of writ- 
ing. So many transmissions at 20,000 volts and thereabouts are now being installed that 
it is almost impossible to keep track of them even by the help of the large manufac- 
turers, through whose courteous assistance this list has been made up. 



INDEX 



Air: 

compression of, efficiency, 51. 
compressor, 48, 50. 
gap in induction motors, 251. 
reheater, 53. 
Alloys, relative properties of, 486. 
Alternating currents: 

characteristics of, 125. 
circuits, properties of, 125. 
compared with d. c, 125. 
Alternating vs. d. c. machinery, 

120. 
Alternators. (See Grenerators.) 
Aluminum: 

conductor joints, 489. 
electrolytic corrosion of, 489. 
vs. copper, 489. 
Anmieters, 661. 
a. c, 663. 
recording, 670. 
sources of error in, 663. 
Ampjere: 

definition of, 21. 
hour meter, 673. 
Analysis of wave form, 169. 
Anchor ice, 409. 
Angle of lag, 131, 133. 

method of measuring, 136. 
Arc motor, 89. 

lamps, power, current, and candle 

power of, 592. 
lighting, commutating apparatus 
for, 285. 
Armature : 

(a. c.) iron clad, 162. 
(a. c.) loss of e.m.f. in, 164. 
four coil drum, 78. 
inductance, ways of reducing, 
165. 



Armature, continued. 

of 5,000 h. p. Niagara generator, 

180. 
reaction, 96, 167. 

effect of, 168, 170. 
slots (a. c), arrangement and in- 
sulation of, 163. 
winding, bar type, 81. 

comparison of Gramme and 

drum types, 81. 
Gramme type, 80. 
iron-clad drum type, 82. 
modem ring type, 82. 
polyodontal, 179. 
principle of, 78. 
turns per coil, 79. 
Arresters, 569. 
Auto-converter, 249. 
Auto-starter, 249. 



Baum's method of alternator regu- 
lation, 197. 
Barlow's wheel, 237. 
Barometeric height effect on strik- 
ing distance, 497. 
Battery, installation of, in water- 
power plant, 635. 
Belting, loss in, 64, 427. 
Boiler capacity for engines, 322. 
Boilers, 309. 

classification of, 325. 

efficiency of, 328. 

evaporating power of various 

types, 330. 
firing of, 331. 

fire-tube vs. water-tube type, 331. 
forcing of, 329. 
fuels for, 329. 



707 



708 



INDEX, 



Boilers, continued. 

furnaces for, 332. 

mechanical stokers, 332. 

merits of different classes, 326. 

results of tests of, 330. 
Booster transformer, 213. 

electrostatic, 521. 
Bradley split phase connection, 215. 
Bridges for damping fluctuations, 

236. 
Bristol voltmeter, 669. 

Cable: 

capacity of, 150. 

for long spans, 548. 

high tension underground, 5^7. 

insulation of, 485. 

methods of locating faults in, 685. 

submarine, 550. 
California Gas and Electric Co. sys- 
tem, 693. 
Canals, construction of, 405. 
Capacity: 

for splitting phases, 215. 

in actual circuits, 150. 

in circuits, 143. 

of armored cables, 150. 

of overhead circuits (formulas 
and curves), 516, 517. 

unit of, 144. 
Catenary curve, formulas for, 540. 
C.G.S. system, 2. 
Chapman regulator, 457. 
Charge, electric, definition of, 9. 
Charging current for line (formula), 

517. 
Circuit breakers, 463. 

with time limit relay, 464. 
Circuits : 

a. c. inductance, 130. 

a. c. phase displacement, 131. 

a. c. properties of, 125. 

angle of lag, in 133. 

method of measuring, 136. 

capacity and inductance in actual 
circuits, 150. 

capacity in, 143. 



Circuits, continued. 

carrying leading current, 146. 
coefficient of self-induction of, 137. 
condensance in, 145. 
effect of energy losses on phase 

position, 153. 
energy losses in, 153. 
impedance, 134. 

diagram with condensance, 149. 
impedances in parallel, 142. 

in series, 141. 
increase of e.m.f. by conden- 

ance, 154. 
inductive e.m.f. 's in, 133. 
power factor in, 139, 149. 
resonance, 155. 
time constant of, 155. 
Circular coil, definition of, 508. 
Clearance in induction motors, 251. 
Coal, as fuel, 24. 

fields, extent and capacity of, 24. 
per i. h. p. with various types of 

engines, 332. 
utilization of, 32. 
Coefficient of self-induction, defi- 
nition of, 137. 
Combe-Garot constant current 

transmission system, 109. 
Conmiercial problem, 639. 
Commutation, process of, 78. 
Commutator, multi-segment, 78. 
PoUock, 284. 
principle of, 18. 
sparking at, 79. 
synchronizing, 281. 
two-part, 18. 
volts per segment, 79. 
Commutators, rectifying, maximun 

output of, 287. 
Compound alternators, 174. 

wires, 488. 
Compounding arrangement for al- 
ternators, diagram of, 196. 
for inductive loads, 175. 
for various power factors, 176. 
of alternators on inductive load, 
196. 



INDEX. 



709 



Compressed air transmission, 48. 
Compressor, air, 48, 50. 

hydraulic efficiency of, 57. 
Taylor hydraulic, 55. 
Compressors, air, efficiency of, 51. 
Condensance, definition of, 145. 
Condenser, effect of frequency upon 
current received and deliv- 
ered by, 144. 
nature of, 143. 

used to increase power factor, 152. 
used to increase e.m.f., 154, 
Conductivity of various metals and 

alloys, 486. 
Conductors, 539. 
compound, 488. 

high tension underground, 576, 
loss of energy in, 475. 
Connections commonly found in 

practice (table), 450. 
Constant current plants in Genoa, 

104. 
Continuous current, 17. 
production of, 77. 
vs. a. c. machinery, 120. 
Converter, mercury vapor, 304. 
cascade, 308. 
efficiency of, 307. 
for constant current, 307. 
Copper, hard drawn, tensile 
strength of, 540. 
losses, 200. 

required by various transmis- 
sion systems, 186. 
vs. aluminium, 488. 
wire, mechanical constants of, 539. 
wire, properties of (table), 509. 
Corliss valve gear, 313. 
Cosines, sines, and tangent (table 

of), 522. 
Cost formula, 510. 
Cotton mill drive, 67. 
Counter e.m. f., 87. 
Cross-arms, 552. 

steel and iron vs. wooden, 562. 
Culm, utilization of, 33. 



Current : 

continuous, 17. 

electric, 10. 

generation of polyphase, 177. 

leading, 146. 

monophase, 158. 

polyphase, 158. 

reorganizers, definition of, 280. 

three-phase, 182. 

transformers, 664. 

imit of, 21. 

value of polyphase for motor pur- 
poses, 179. 
Currents (a. c), characteristics of, 

125. 
Cycle, definition, 136. 

Dampers : 

on synchronous machines, 236. 
Damping, 235. 
Dams, 399. 

concrete steel, 404. 

construction of, 399. 

masonry, 401. 

materials for, 400. 

timber, 402. 
D'Arsonval galvanometer, 662. 
Delta connections, 185, 209. 
Depreciation charges, 647. 
Dielectric constant, definition of, 

143. 
Discounts, 656. 
Distribution : 

arc lighting, 591. 

centre of load, 621. 

constant potential, 119. 
efficiency of, 120. 

current and power taken by 
lamps, 592. 

desirability of motor service, 598. 

diphase system, 613. 

direct from transmission circuit, 
581. 

efficiency of, 63. 

example of, 440. 

substation system, 624. 

few vs. many transformers, 587. 



710 



INDEX. 



Distribution, continued. 

from eccentrically located station, 
603. 
large reducing stations, 607. 
scattered substations, 600. 
substation, 621. 

heavy substation, 629. 

interconnected diphase system, 
615. 

interdependent dynamos and 
motors, 115. 

maintenance of uniform voltage, 
583. 

methods of, 581. 

monocyclic system, 612. 

monophase system, 611. 

motor generator device to com- 
pensate for losses, 606. 

motor power, 62. 

motor service, 590. 

of power by shafting, belts, etc., 65. 

polyphase system, 613. 

primary, 590. 

problem, 482, 621. 

radial from centrally located 
station, 600. 

radius of operation of trans- 
formers, 590. 

railway load in addition to motor 
and lighting service, 602. 

regulation of voltage on secon- 
dary lines, 630. 

relative importance of polyphase, 
heterophase and single-phase 
systems, 277. 

secondary mains, 589. 

substation vs. house-to-house, 585. 

three-phase system, 615. 

three-wire system, 114, 610. 

two-wire system, 610. 

voltage, 608. 
Doble water-wheel, 362. 
Draft tube, 351. 
Drive: 

choice of, 427. 

from vertical shafts, 430. 
Dynamos. (See Generators.) 



Dynamotor, 103. 
D5Tie, definition of, 20. 

Eddy current loss, 201. 
Edison three-wire system, 113. 
Electric charge, definition of, 9. 

current, definition of, 10. 

current, propagation of, 

transmission. (See Transmission 
Electricity: 

flow of, 7. 

nature of, 1. 

principles of, 1. 

static, 8. 
Electro-magnetic induction, 13. 

strains, 11. 
Electrolytic strain on insulation^ 

123. 
Electrostatic booster, 521. 

instruments, 683. 
E.M.F. automatic regulation of 
polyphase generators, 194. 

effective, 188. 

generation of, 127. 

impressed, 131. 

increase of, by use of condenser, 
154. 

induced, direction of, 14. 

inductive, 131. 

in resonant circuit, 156. 

loss in a. c. generator, 164. 

teaser, 189. 

imit of, 20. 

waves, 128. 
Energy: 

apparent, 136. 

classification of, 4. 

conservation of, 3. 

definition of, 2. 

electrical, 7. 

electrical measurement of, 660. 

internal heat of earth, 31. 

losses, effect on phase position 
of current, 153. 

luminous, 5. 

measurement of, on three-phaae 
circuit, 676. 



INDEX. 



711 



Energy, continued. 
potential and kinetic, 2. 
sources of, 24. 
transformation of, 3. 
transformation, efficiency of, 4. 
wave, 5. 
Engine : 
and dynamo, combined efficiency 

of, 64. 
choice of, for given service, 324. 
steam, boiler capacity necessary, 
322. 
boiler pressure, 318. 
choice of, for power service, 320. 
choice of, for railway service, 

320. 
classification, 312. 
coal per i.h.p. for various types 

of, 332. 
compound vs. simple, 315. 
condensing vs. non-condensing, 

315. 
effect of varying load on econ- 
omy, 319. 
performance at different loads 

of various types, 321. 
performance of, 333. 
piston speed, 316. 
principle of, 309. 
speed of, 325. 
steam consumption, of different 

types, 318. 
thermal efficiency, expression 

for, 310. 
use of, superheated steam in, 

322. 
valves, 312. 
Engines, 309. 
gas, 323. 

cost of fuel for, 642. 
cost of operation, 642. 
economy of, 25. 
thermal efficiency of, 323. 
solar, 27. 
Ether, 5. 

Evaporation, definition of, 329. 
in various types of boilers, 330. 



Exciter equipment, 455. 
Exciters, choice of, drive for, 432. 
connection of, 456. 

Faesch & Piccard governor, 380. 
Farad, definition of, 144. 
Faults, method of, locating, 685. 
Field: 

about current carrying corw 
ductor, 11. 

distortion of, 107. 

windings, 83. 

compound type, 85. 
series type, 84. 
shunt type, 84. 
Fire risk of transformers, 447. 
Fire-tube boilers, 326. 
Flat rate, 655. 
Flume, timber, 439. 
Flumes, 357. 

loss of head in, 397. 
Frazil, 409. 
Frequency: 

choice of, 278. 

formula for, 160. 

indicator, 471. 

meter, 670. 

used in rotaries, 300. 
Fuel: 

coal, 24. 

gas, 24. 

oil, 334. 

variation of cost of, throughout 
day per k.w., 348. 
Fuels, heat of combustion and 
evaporative power of va- 
rious, 329. 
Furnaces for boilers, 332. 
Fuses, 461. 



Galvani constant current 

104. 
Galvanometer, 662. 
Gas as fuel, 24. 
Gas engine. {See Engine.) 

economy of, 25. 
Gearing, bevel, loss in, 41. 



plant, 



712 



INDEX. 



G. E, voltage regulator, 458. 
Generator and engine, combined 

efficiency of, 64. 
Generators : 

a. c, armature reaction, 167. 

as effected by inductance, 523. 

Baum's method of regulation, 
197. 

compound wound, 85, 174. 

compounding for inductance 
loads, 175. 

connections commonly found 
in practice, 450. 

constitutional features of, 159, 

device for over-compounding 
on inductive load, 195. 

direct connection of, 193. 

efficiency of, 59. 

field. {See Field.) 

formula for frequency, 160. 

G. E. compensated field alter- 
nator, 196. 

general construction of, 190. 

heterophase, 189. 

inductor type, 192. 

methods of reducing induct- 
ance in, 165, 166. 

monocyclic system, 188. 

polyphase, regulation of, 194, 

practical limits of voltage, 
532. 

principle of, 126. 

regulation of, 173. 

relation between poles, speed, 
and frequency, 160. 

revolving field, 191. 

revolving field, advantages of, 
194. 

series wound, 84. 

shunt wound, 85. 

star and delt connections, 184. 

theoretical e,m.f . generated by, 
164. 

three-phase, 181. 

three-phase, efficiency of, 194. 

wave forms, 128. 

windings, 161. 



Generators, continued. 

advantages of moderate voltage, 
441. 

arrangement of, in power station, 
432. 

choice of drive, 427. 

comparison of a. c. and d. c, 18. 

commutators. (See Commutator.) 

cost of, 651. 

design, principles of, 18, 19. 

energy required for excitation, 
455. 

high voltage, 689. 

high voltage, d. c, 109. 

inductance of, 150. 

insulation of, from floor, 441. 

location of, 424. 

operated in parallel, 442. 

principle of, 14. 

regulation of compounding, 116. 

turbo, 342. 

two-phase, 178. 
Glass vs. porcelain, 492. 
Governors: 

action of, 371. 

classification of, 372, 

Fsesch and Piccard type, 380. 

hydraulic, disadvantages of, 385. 

load type, 374. 

Lombard type, 376. 

on Pelton wheel, 384. 

water-wheel, 370. 
Replogle type, 382. 
Gramme ring, 80. 
Ground detector, 472. 
Grounded conductors for light- 
ning protection, 574. 

neutrals, 450. 
Gutta-percha, 502. 

Heat: 

radiant, 5. 

of combustion, definition of, 329. 
fuel oil, 334. 
Heating value of various fuels, 329. 
Henry, definition of, 138. 
Heterophase systems, 189. 



INDEX, 



713 



High Voltage: 

measurement of, 665. 

measurements, 683. 
Hoist motor, 94. 
Huntng, 233, 235. 
Hydraulic : 

development, 387. [415. 

maximum allowable, cost of, 

plants, description of various, 419. 

power, price of, 43, 46. 
Hydro-electric plant, efficiency of, 

67. 
Hysteresis losses, 200. 

Ice: 

on wires, 543. 
Idlers for rope drive, 38. 
Impedance : 

definition of, 134. 

diagram, 135. 

factor, 511. 
Impedances: 

addition of, 15. 

in parallel, 142. 

in series, 141. 
Incandescent lamps: 

220 volt, 608. 

watts per candle-power, 593. 
India rubber, 502. 
Individual drive, 62. 
Inductance : 

armature, ways of reducing, 165. 

effects on generator, 523. 

for splitting phases, 216. 

in actual circuits, 150. 

line, 506. 

nature of, 130. 

of generators and transformers, 
150. 

of generators, method of redu- 
cing, 166. 

of line (curves), 514. 

on line, 121. 

troubles caused by inductive 
drop, 141. 

unit of, 138. 

used to preserve regulation, 523. i 



Induction: 

electromagnetic, 13. 
motor, 237. 

advantages of, 266. 
arrangement of windings, 253. 
auto starter, 249. 
choice of, 276. 

comparative qualities of differ- 
ent types, 270. 
construction of, 239. 
depth of air gap, 251. 
form of slots, 253. 
maximum torque, 273. 
performance curves, 266, 268. 
primary winding, 247. 
principle of, 239, 244. 
relation between static and 

running torque, 274. 
relation between resistance and 

reactance in, 273. 
secondary winding, 239. 
single-phase, 258. 
single-phase, characteristic 

curves, 260, 262. 
single-phase, principle of, 255. 
slip as affected by resistance, 

273. 
slip in, 241. 
slow speed, 268. 
speed regulation, 275. 
starting current, 271. 
starting torque, 271. 
use of resistance, in secondary, 
272. 
wattmeter, 673. 
Inductive drop, 141. 
Inductor type alternator, 192. 
Instrument equipment of generat- 
ing station, 666. 
Instruments : 

continuous current, 660. 
edgewise type, 473. 
electrostatic, 683. 
used in power transmission, 
468. 
Insulated wires, classification of, 
502. 



714 



INDEX. 



Insulation: 

continuous, 501. 

materials available for, 501. 

of a. c. and d. c. lines, 121, 123. 

of a. c. armature slots, 163. 

of bar-wound armatures, 81. 

of constant current line, 109. 

of lines, 122. 

of machines operated on constant 
current line, 101. 

tests, 685. 
Insulator pins. {See Pins.) 
Insulators : 

factor of safety, 499. 

for high tension work, 565. 

line, 491. 

number replaced yearly, 568. 

porcelain, 565. 

sparking, distance for, 567. 

support of, 556. 

strain (novel type), 549. 
Interrupter static, 572. 

Joule : 

definition of, 21. 

Kelvin : 

balance, 683. 
Kelvin's law, 478. 

modifications of, 479. 
Kinetic energy, 2. 

Lahmeyer rotary, 294. 

Lamps, 220 volts, 608. 

Leakage, 491. 

Lentz's law, 16. 

Light, electromagnetic theory of, 6, 

energy, 5. 
Lighting, lamps in series, 102. 
Lightning, 568. 

arresters, 569. 

danger from — with a. c. and 
d. c. apparatus, 123. 

protection, grounded wire, 574. 
Line, 474. 

amount of copper required, 476. 



Line, continued. 

calculation of terminal voltage, 

517. 
calculations of losses, etc., 508. 
capacity of (formula and curves), 
516, 517. 

charging current (formula), 517. 

choice of initial voltage, 531. 

conductors, 539. 

conductors, loss in, 475. 

construction used on Missouri 
River Power Co., 559. 

continuous insulation of, 501. 

cost formula, 510. 

energy losses in (curves), 496. 

entrance into buildings, 575. 

(erected), cost of, 651. 

formula for self-induction in, 511 

formula for weight of wire re- 
quired, 509. 

grounded wires for lightning pro- 
tection, 574. 

impedance factor, 511. 

inductance in, 506. 

insulation, 490. 

insulators. (See Insulators.) 

its general relation to the plant, 
474. 

junctions between cables and 
overhead lines, 577. 

lightning arresters on, 574. 

lightning stroke, 569. 

long, cost of maintenance, 688. 

long spans, 698. 

loss of current to earth, 491. 

maximum loss in, 534. 

mil-foot constant, 509. 

overhead, 505. 

pins. (See Pins.) 

poles. (See Poles.) 

provision for repairs, 578. 

river-crossings, 550. 

skin effect, 515. 

static disturbances, 530. 

steel towers, 547. 

surging, 528. 

telephone, 578. 



INDEX. 



715 



Line, continued. 

three-phase, formula for weight 

of wire, 510, 
tower construction, 562. 
tower, total cost of, 563. 
towers, cost of, 563. 
voltages, 499. 
wave form, 127. 

way of treating inductance, 511. 
wire, 539. 

choice of deflections, 546. 
copper, mechanical constants 

of, 539. 
deflection due to temperature, 

542. 
factor of safety, 544. 
ice loaded, 543. 
maximum deflection of, 541. 
maximum length of span, 544. 
relation between deflection, 
tension, and length of span, 
546. 
wind-pressure on, 543. 
wires, transposition of, 558. 
Lines, duplicate, 485. 

general character of, 483. 
Load governors, 374. 
lines (curves), 347. 
synchronous motor, disturbing 
effect of, 171. 
Lombard governor, 376. 
Loop test, 685. 
Los Angeles Co., system, 695. 



Magnet, solenoid, 12. 
Magnetic field, 11. 
Market, estimate of, 639. 
McCormick turbine, 356. 
Measurements, electrical, 660. 
Mechanical drive, efficiency of, 65> 
Mercury rectifier in arclighting, 597. 

vapor converter, 304. 
Mershon's tests, 497. 
Mesh connections, 185, 209. 
Metals, relative properties of, 486. 



Meters, chemical, 682. 
reading of, 681. 
testing of, 680. 
Microfarad, definition of, 144. 
Miner's inch, definition of, 391. 
Monocyclic system, 188. 
Motor : 

generator, 103, 288. 
advantage of, 289. 
d. c. loss in, 117. 
disadvantages of, 290. 
efiiciency of, 290, 302. 
efficiency of large sizes, 292. 
synchronous, operation of, 221. 
in transmission, 221. 
maximum power factor, 225. 
output, input, etc., of, 223. 
principles of, 217. 
vector diagram of, 224. 
water, 44. 

hydraulic efficiency of, 44. 
impulse, 45. 
oscillating, 44. 
Motors : 
electric, 
a. c, 217. 
arranged for wide speed range, 

100. 
classification of operating con- 
ditions, 89. 
commutating a. c. {See Series 

a. c.) 
commutator. (*See Commutator.) 
compared with mechanical 

drive, 66. 
constant speed series, 95. 
cost of — installed ready to 

nm, 644. 
current taken by, 87. 
differential shunt motor, 99. 
drive, choice of, 62. 
effect of synchronous — on 

wave form, 171. 
efficiency of, 59. 
efficiency of system, 63. 



716 



INDEX. 



Motors, continued. 

efficiency for different sizes 

(table), 62. [load, 63. 

efficiency, variation of, with 
field. {See Field.) 
fundamental principle, 237. 
high voltage, d. c, 109. 
induction. {See Induction 

Motor.) 
installation of — for constant 

current systems, 107. 
interpole, 121. 
performance of, 87. 
principle of, 14, 17. 
pull on armature conductors, 

'86. 
self-regulating series, 94. 
series a. c. {See series, Motor.) 
series driven by series dynamo, 

95. 
series for constant potential, 

91. 
series-wound constant current, 

89. 
shunt-wound constant poten- 
tial, 96. 
single -phase {See Single-phase 

Motors.) 
synchronous. {See Syncliro- 

nous Motor.) 
torque at armature surface, 86. 
voltage of, 113. 
with one meter, 679. 
working of, 86. 
wave, 31. 

Needle-valve for water-wheels, 363. 
Nemst lamps, 597. 

Ohm, definition of, 21. 

Ohm's law, 475. 

Oil fuel, 334. 

Overload circuit-breaker, 463. 

Pacinotti constant current plant, 
106. 

Parallel, operation, switching re- 
quirements for, 461. 



Paralleling of alternators, 443. 
Pelton wheel, 44, 352. 
governing of, 384, 
Pendulum, oscillation of, 155. 
Periodicity. {See Frequency.) 
Permutator, 309. 
Petroleum as fuel, 24. 
Phase displacement, 131. 

lamps, 443. 
Pilot wires, 457. 
Pins, 556. 

burning of, 560. 

composite, 561. 

metal, 561. 

treated, 561. 
Pipe line: 

concrete, 697. 
cost of, 418. 

lines, construction of, 406. 
loss of head in, 397. 

steel hydraulic, properties of 
(table), 408. 
Plant, location of, 23. 
Plants, in parallel, 461. 
Pneumatic transmission. {See 

Transmission.) 
Pole-head, used on Niagara-Buffalo 
line, 558. 

line, life of, 574. 
cost of, 652. 
Poles, 551. 

at angles, 554. 

bending moment of, 553. 

classification of, stresses on, 553. 

creosoting of, 552. 

cross-arms, 552. 

crushing resistance of, 553. 

general dimensions of, 551. 

guying of, 554. 

number per mile, 553. 

stresses from sleet-storms, 556. 

wind-pressure on, 555. 

wood for, 551. 
PoUak commutator, 284. 
Porcelain vs. glass, 492. 
Potential energy, 2. 

transformer, 665. 



INDEX. 



717 



Power: 

centralization of, 33. 
cost at customer's meter, 647. 
cost at switchboard, 647. 
cost of, wlien developed by va- 
rious types of steam-engines, 
640. 
cost of, when developed by divers 

engines, 643. 
cost of per k.w. for different 

capacities, 646. 
definition of, 35. 
determination of price of, 654. 
estimate of cost of, 639. 
estimate of market for, 416. 
factor, 149. 

definition of, 139. 
increase of, with condenser, 152. 
indicator, 471. 
plant, choice of power units, 
426. 
load curves, 347. 
organization of, 418. 
station. {See Power-station.) 
transportation of materials for, 

426. 
variation of cost of fuel per 
k. w. throughout day, 348. 
plants, description of various, 
419. 
list of — operating at more 
than 20,000 volts, 702. 
station, at Folsom, Cal. 
at Fresno, Cal., 436. 
building, 425. 
design of, 418. 
foundations for, 422. 
general arrangement of typi- 
cal station, 434. 
lighting-arrester system for, 

574. 
location of, 418. 
location of high voltage wires, 

453. 
location of generators, 424. 
number of units, choice of, 
429. 



Power, continued. 

of Truckee River, G. E. Co., 

438. 
operated in parallel, 693. 
reserve apparatus, 636. 
structure, 423. 

switchboard. {See Switch- 
board.) 
traveling crane in, 454. 
steam, cost of, 415. 
steam electric, cost of, 69. 
Prime movers, classification of, 
309. 
gas-engines. {See Engines.) 
steam-engine. {See Engine.) 
steam-turbines. {See Turbines.) 
water-wheels. (5ee Water-wheels.) 
Pumping, 233, 235. 

Railway : 

a. c. transmission d. c. distri- 
bution efficiency of system, 
111. 
motor, 91. 
Railways with three-wire system, 

114. 
Rainfall observations, 394. 
Rates, determination of, 654. 
Ratio of transformation, definition 

of, 200. 
Reactance, negative, 145. 
Rectifiers, 280. 
commutating, 280. 
electrolytic, 303. 
Rectifying commutator, advantages 
of, 287. 
commutators, maximum output 
of, 287. 
Regulation : 

Baum's method, 197. 

best modern practice, 692. 

close, 302. 

diagram, 519. 

of alternators, 173. 

of alternators with inductive 

load, 174, 177. 
of polyphase generators, 194. 



718 



INDEX. 



Regulation, continued. 

of three-phasers, 184. 

of voltage, 456. 

of voltage on secondary lines, 630. 

of water-wheels, 370. 

preserved by use of inductance, 
523. 

speed. {See Speed.) 
Regulator: 

Chapman type, 457. 

G. E. type, 458, 632. 

Stillwell type, 631. 
Regulators for constant current, 

principle of, 593. 
Relay, time limit, 464. 
Reorganizers, definition of, 280. 
Replogle governor, 382. 
Resistance : 

apparent, 134. 

increase of — by alternating cur- 
rent, 515. 

of copper wire, 509. 

unit of, 21. 
Resonance, 170. 

as affected by armature reaction, 
168. 

dynamics of, 155. 

testing for, 168. 
River crossings, 550. 
Rivers: 

low land, 388. 

measurement of flow, 392. 

mountain, 388. 

slow, 387. 

swift, 387. 

upland, 388. 
Rope drive, cost of, 41. 

cost of plant, 70. 

cost of plant operation, 70. 

efficiency of, 38. 

idlers, use of, 38. 

losses in, 427. 

multiple sheaves, 38. 

multiple sheaves, efficiency of, 
39. 

power, size of rope, diameter of 
pulley and speed (table), 42. 



Rope drive, continued. 
straightaway, 37. 
wire, 35. 

construction of rope, 36. 
efficiency of, 42. 
span, 36. 
speed of, 36. 
Rotary converter. {See Synchro- 
nous Converter.) 
Rotating field, 240, 244. 

Samson turbine, 354. 
Scott system of connections, 210. 
Self-induction in a circuit, 511. 
Series motor: 

a. c, 262. 

commutation sparking, 265. 

compensating winding, 264. 

efficiency and power factor of, 
265. 

Westinghouse, 264. 
Shafting, losses in, 65. 
Shallenberger meter, 673. 
Sheaves, rope, construction of, 37. 
Sheefer meter, 673. 
Shell-boilers, 326. 
Shields, for damping fluctuations, 

236. 
Sines, tangents, and cosines, table 

of, 522. 
Single-phase motors, characteristic 
curves, 260, 262. 

efficiency and power factor, 259. 

induction. (/See Induction motors.) 

power factor of, 277. 

uses of, 258. 

Wagner, 259. 
Skin effect, 515. 
Slip in induction motor, 241. 
Slots in induction motors, 253. 
Solar-engines, 27. 

cost of, 28. 
Speed : 

constant — motor, 95, 97. 

of engines and dynamos, choice 
of, 3'25. 

regulation for series motors, 93. 



INDEX, 



719 



Speed, continued. 

regulation of constant-current 
series motors, 90. 
of induction motors, 275. 
of shunt-motor, 98. 
variable — motor, 92, 93, 98. 
wide-range, motor, 100. 
Split-phase connections, 215. 
Squirrel cage, secondary, 239. 
Stanley motor, 255. 
Star connection, voltage to neutral, 
449. 
connections, 185, 209. 
Static, 530. 
electricity, 8. 
interrupter, 572. 
Steam and water-power, relative 
cost of, 33. 
auxiliary, 412. 
consumption of, different types 

of engines, 318. 
electric plant, cost of, 69. 

efficiency of, 66. 
engine. {See Engine.) 
gauges, recording, 670. 
plant, cost of, 651. 
power of, cost of, 415. 
superheated, use of, 322. 
turbine. {See Turbine.) 
Stillwell regulator, 631. 
Stokers, mechanical, 332. 
Strain insulators, novel, 549. 
Street lighting, 593. 
Strength of various metals and 

alloys, 486. 
Striking distances, 494. 
Substation : 

reserve apparatus in, 636. 
Surging caused by lightning, 569. 
definition and theory of, 528. 
e.m.f. of, 529. 

relation of voltage rise to cur- 
rent broken, 530. 
Switchboard apparatus, 455. 
equipment of panels, 469. 
location of, 451. 
purpose of, 459. 



Switchboards, 455. 
Switches: 

air-break for high voltage, 467i 
electrically operated, 464. 
for remote control, 464. 
oil-break, 462. 
Switching connections, elementary, 

460. 
Synchronous converter, 293. 
action in, 297. 
and transformers, combined 

efficiency of, 302. 
connection of, 188. 
effects of line loss, inductance 
and resonance upon d. c. 
e.m.f., 301. 
efficiency of, 300. 
frequencies used, 300. 
in railway operations, 299. 
ratio between a. c. and d. c. 

e.m.f., 301. 
winding of, 297. 
motor, advantages of, 229. 
disadvantages of, 230. 
hunting or pumping of, 233. 
load, disturbing, effect of, 171. 
minimum, practical size of, 

233. 
power factor of, 228. 
polyphase power factor of, 232. 
polyphase, 232. 
regulation of line by, 227. 
self-starting, 231. 
starting of, 230. 
uses of, 277. 
with solid poles, 235. 
Synchronism, definition of, 218. 

indicators, 443. 
Synchronization, automatic, 470. 
Synchronizing commutator, 281. 
Synchronscope, 469. 



Tangents, sines and cosines, table 

of, 522. 
Taylor hydraulic air-compressor, 55. 
Teaser, 189. 



720 



INDEX. 



Three-wire system, 113. 

for railways, 114. 
Thury system, 109. 
Tidal energy, 28. 

cost of utilizing, 30, 
Time, constant of, electric circuit, 

155. 
Torque : 

constant, 89. 

maximum in induction motor, 

273. 
relation between static and run- 
ning in induction motor, 
274. 
Towers, 562. 
Transmission : 

a. c. and d. c. compared, 121. 
analysis of, 68. 

comparison of commercial pos- 
sibilities of different systems, 
68, 76. 
comparison of rope and electric, 

39. 
electric, a. c. 158. 

a. c, classification, 158. 
a. c, material of, 159. 
at high voltage, present ten- 
dencies, 687. 
best system for heavy sub- 
station work, 634. 
constant current, 101. 
constant potential. 111. 

voltage control, 112. 
continuous current voltage 

control, 103. 
copper required by various 

systems, 186. 
cost of operation, 71. 
cost of plant, 71. 
d. c. system, 77. 
development of network, 689. 
delivery known power from 

limited water-power, 482. 
delivery of known power from 

ample water-power, 481. 
effect of distance and voltage 
on copper required, 476. 



Transmission, continued. 

efficiency of, 110. 

efficiency of, at full and half- 
load of different systems, 74. 

efficiency of system, 61, 66. 

general distribution from water- 
power, 481. 

heterophase systems, 189. 

installations, 702, 703. 

line efficiency, 61. 

line insulation. (^See Insulation.) 

lightning protection. (See 
Lightning.) 

longest distance, 687. 

monocyclic system, 188. 

polyphase, efficiency of. 111. 

polyphase, with rotary con- 
verter, efficiency of. 111. 

problems, 480. 

study of various cases, 481 . 

synchronous motor for regu- 
lation, 228. 

the line. (See line.) 

voltage to be used, 477. 

vs. all other systems, 58. 

undergroimd, 484. 
gas, 58. 

gearing bevel, efficiency, 41. 
general conditions of, 23. 
hydraulic, 42. 

allowable velocity in pipes, 46. 

efficiency of, 44, 47. 

high artificial pressure, 45. 

loss of head in pipe (table), 47. 

medium pressure, 43. 
methods, classification of, 35. 
of coal energy, efficiency of, 32. 
pneumatic, 48. 

allowable velocity, in pipes, 52. 

cost of plant, 70. 

cost of operation, 70. 

efficiency of, 54. 

loss of head in pipes (table), 
52. 

Paris system, 54. 

price of power, 54. 

process of, 50. 



INDEX. 



721 



Transmission, continued. 
reheater, use of, 53. 
rope. {See Rope Drive.) 
straightaway, 37. 

cost of plant, 70. 

cost of operation, 70. 
shafting, belting, etc., losses in, 

65. 
sphere of application of different 

systems, 75. 
wire-rope, 35. 
efficiency of, 38. 
Transformers, 198. 

a. c. to d. c, choice of apparatus, 

303. 
air-blast, 448. 
artificial cooling of, 204. 
choice of, 446. 
connection of, 208. 
connected for split single-phase 

into three-phase, 215. 
connections commonly used in 

practice, 450. 
constant current, 594. 
constant loss in, 587. 
construction of, 199. 
core type, 203. 
cost of, 651. 
current, 664. 
data, 201. 
determination of magnitude of 

units, 445. 
duplex machine. {See Syn- 
chronous Converter.) 
efficiency of, 59, 201, 205, 206, 

588. 
fire protection, 447. 
fire risk in, 447. 
high voltage, location of, 448. 
inductance of, 150. 
installation of, 451. 
losses in, 200. 
maximum, practicable voltage of, 

687. 
maximum, size of , self-cooled, 446. 
motor-generator. {See Motor 

Generator.) 



Transformers, continued. 
polyphase, 206. 
principle of, 130. 
radius of operation of, 590. 
ratio of transformation of, 200. 
rectifiers. {See Rectifiers.) 
rotary converter. {See Syn- 
chronous Converter.) 
shell type, 202. 
star, mesh and resultant mesh 

connections, 209. 
static converter. {See Converter.) 
two to three-phase and vice versa, 

210. 
two to three-phase and vice versa, 
without special transform- 
ers, 212. 
used as boosters, 213. 
working of, 199. 
Turbines: 
steam, 334. 
actual efficiency of, 345. 
advantages of, 344. 
Curtis, principles of, 342. 
De Laval, principles of, 335. 
De Laval, steam consumption 

of, 336. 
Parsons, efficiency of, 340. 
Parsons, performance curves 

of, 340. 
Parsons, principles of, 336. 
Parsons, steam consumption 
of, 340. 
water, choice of, 367. 
for high heads, 360. 
for low heads, 360. 
governors for. {See Gov- 
ernors.) 
impulse type, 351. 
impulse type, efficiency of, 366. 
impulse type, maximum effi- 
ciency of, 361. 
installation of, 357. 
McCormick type, 356. 
methods of regulating, 365. 
multiplex types, 369. [of, 362. 
Pelton and Doble, efficiency 



722 



INDEX. 



Turbines, continued. 

pressure type, 351. 

pressure type, losses in, 364. 

pressure type, efficiency of, 
364. 

Samson type, 354. 

Victor type, 355. 
Turbo-generator, 342. 

Umformer, 294. 
Units, 20. 

Valve: 

needle for water-wheels, 363. 
Valves, engine, dependent type, 
313. 

independent type, 312. 
Victor turbine, 355. 
Volt, definition of, 20. 
Volta, constant current plant, 106, 

108. 
Voltage: 

choice of, initial, 531. 

diagram, 519. 

regulator, of, G. E. Co., 632. 

regulation for lighting and motor 
service, 513. 

rise at end of line containing 
capacity, 521. 
Voltages, striking distance at vari- 
ous, 494. 
Voltmeter relays, 457. 
Voltmeters, 664. 

a. c, 665. 

classification of, 387. 

connections for, 667. 

cost of, 648. 

recording, 669. 

Water-power : 

and steam, relative cost of, 33. 
canals, 405. 

creation of artificial, 699. 
dams. {See Dams.) 
development of, 387. 
development, maximum allowable 

cost of, 415. 
difficulties from ice, 409. 



Water-power, continued. 

distribution of, 25. 

estimate of market for, 416. 

formula for available h. p., 396. 

formula for mechanical h, p., 396. 

measurement of flow, 389. 

mountain streams, development 
of, 399. 

pipe line, cost of, 418. 

plant, itemized cost of, 648. 
cost of generating and trans- 
mitting power, 650. 
operating expenses of, 651. 

protection against ice, 409. 

questions involved in develop- 
ment of, 410. 

rainfall observations, 394. 

reconnoissance of, 389. 

settling tanks for sand, 408. 

steam auxiliary, when to instrll, 
414. 

steel and iron pipe lines, 406. 

storage, when to provide, 410. 

storage reseivoir, 397. 

utilization of, 416. 

varying head, how to deal with, 
368. 

with steam auxiliary, 412. 

wooden pipe lines, 406. 
Water-tube boilers, 326. 
Water velocity, allowable, 46. 
Water-wheels, 349. 

classification of, 349. 

cost of, 651. 

drive from vertical, 430. 

governors. {See Governors.) 

installation of, 357. 

Leffel cascade type, 363. 

Pelton, 352. 

principles oi, 350. 

regulation of, 370. 

timber flumes, 439. 

turbine. {See Turbines.) 
Watt, definition of, 21. 
Wattmeter: 

connection to three-phase circuit, 
676. 



INDEX. 



723 



Wattmeter, continued. 

for two or three-phase circuits, 

678. 
induction type, 673. 
integrating, 671. 
recording, 671. 
Wave energy, 5. 

form analysis of, 169. 

as affected by inductive load, 

524. 
as affected by synchronous 

motor, 171. 
device to obtain sine, 164. 
in practical circuits, 527. 
of three-phasers, 184. 
Wave forms, 128. 



Wave motors, 31. 

Waves, irregular forms of, 278. 

Weston, d. c. instruments, 663. 

Wiers: 

coefficient formula for, 391. 

formula, 390. 

table, 391. 
Wind-power, windmills as prime 
movers, 26. 

pressure on line wire, 543. 
Winding armature. (See Armature 

Winding.) 
Wire-rope, 36. 
Woods, tensile strength of various, 

554. 
Work, unit of, 21. 



SEP 11 190/ 



